I hadn't downloaded Hearthfire yet (But already pretty much paid for it...) and I am working on making gold for my houses... I currently have over 88,000 gold and can prolly make around 10,000 - 20,000 more by selling the rest of my diamonds... How much do you recommend to have for all houses? (Yes, I'm planning to buy all land plots and have mansions! One for each child, Hm?)

5000 for each land, so 15,000, with at least another 20000 = total of 35,000 for all three houses... You can save a ton of money if you manually furnish your houses, also if you actively adventure.

Hearthfire came a little too late for one of my character, so it is a little confusing; but still fun since I now dedicate an entire house for displaying random stuff. I found that starting a brand new character allows you to get involve and customize your houses much deeper.

For one house, what's the maximum possible cost involved?

Honestly?...15,000 maximum, if you manually furnish everything and get materials by yourself. (Aside from the clay, wood and stone; those alone can cost you 5,000 per house).

5,000 can probably also cover glass, straws, and goat horn cost. So minimum you should start with 10,000 per house and go from there.

Well.. I currently have over 108,000... I am planning to get one house and do what I truly wish with that one house and build everything I want in it.. So most likely that house will have the most money spent in it.. Thing is, how much?

I mean, this isn't going to be like RuneScape where I spent 50 Million+ on my house XD, that is for sure.

Well, the two characters I know are going to build have 42,000 and 160,000 septims respectively. Should be more than adequate.

Not sure about my Khajiit yet... since he is unlikely to marry or adopt, he might not see the point in it. It'll probably depend on how fun house-building is.

You can get wood, clay and stone for free, there are plenty of mining opportunities around Skyrim so you should be able to get the metal for free as well. Five thousand for the land you could build the house for free, a bit of hunting for your trophies and some leather and the remaining items are not that expensive. How much you spend depends on how much work you want to put into it. Straw and glass are not that expensive, you could have picked up whatever else you need raiding and adventuring.

You don't need much gold, you need about 200-300 iron ingots. You need about 50 corundum ingots. And you will want to buy up as much glass and horns at possible for display cases and lights.

138,000 builds 3 homes easily

On my experience, around 15-20,000. I really went nearly all out on lakeview manor.

I knew about the ingots. My Nord and her bodyguard (and probably future husband) are carrying about 75 each. Good thing I leveled her Stamina.

Do we need anything else that's available in the vanilla game?

Yeah, the iron ingots are my main problem as I keep running out of them and most blacksmiths don't have more than 10 or so. Also, just a few moments ago I depleted my strategic reserves (=70 ingots I kept at Breezehome just in case)

Not a large quantity of anything. You will need a few grand soul gems for an enchanting table, and I think a few quicksilver and moonstone ingots. I think you will need about 50 leather strips too. It all depends on how many houses you intend to build and also, very specifically how much you intend to decorate each of those houses.

Oh yeah, and the trophies in the trophy room have individual required items, so the trophy room will depend on which trophies you wish to display.

So far I have spent, all in, 10,000 Septims, but that includes the plot as well. I've built all the wings, and I've fully furnished the bedroom, the library and almost all of the main hall. I still have the Storage room and the entry hall to do as well as the Cellar.

all the gold in skyrim is what you will need trust me all those logs add up fast i wager a fully furnished hearthfire manor costs about twice maybe 3 times as much as the most expensive house in the game when all is said and done.

You will need amulets of the divines if you want to build a shrine to each divine in the basemant. each shrine requires 1 amulet of its respective divine, and you can build shrines in all 3 houses.

I can buy the materials I can't find, right? I've heard horror stories about glass and goat horns...

I'm building only Talos. Don't really need any other divine shrines.

Killing a merchant resets their inventory upon reload (or if they're marked essential, just putting them in recovery state works too). I buy all the iron ingots the blacksmith has, save my game, kill the blacksmith, reload my game, buy again, wash, rinse, repeat.

Glass, straw and Goat horns are all available from the general goods vendors (Belethor, Arnleif and sons, etc). They each sell in bundles of around 10-20 pieces. Depending on how many display cases you build, you will need the most glass, probably at most 100 pieces. And the Goat Horns are for the candle sconces, I wanted every possible candle, so I think I needed about 40-50 for 1 full house.

The straw is very limited in its use, so you won't need much straw at all.

You've more than enough if you intend to build many things yourself. The only things you'll need to buy, because they're very scarce in quests, are glass, straw, and goat horns (in fact, I've not seen goat horns anywhere but shops). If you're looking to "put everything I can" into each room, then the grand total you'll spend, per house, is 7000g (5000g for the plot, 2000g for glass, straw, and goat horns and you'll have change left over).

You can saw your own logs at a mill. Be sure to chop firewood and sell it back first, though, or you won't get the option to do so.
You'll need lots and lots of iron ingots, and I've burned through 250 in one house without blinking an eye. These are for the nails, fittings, and hinges, as well as the locks, which also require corundum ingots. Each add-on (with doors) will require 2 locks, so that's 6 minimum. Each chest requires a lock as well, and chests vary in number by add-on. In my Falkthreath house, which has everything, I'm pretty confident I used nearly 50 corundum ingots, because I added safes (which require steel ingots).

Arcane and alchemy tables will require gold, a grand soul gem (filled), and other ingots to build.

Now, if you intend to mine ore, it'll take you some time to acquire the necessary minimum, as there are only a limited number of deposits in Skyrim. They do respawn, but not for quite some time. This means if you want an immediate complete house, you will have to buy ore/ingots (I recommend buying ore only, and smelter them yourself).

Clay and stone can be found at every house location, within steps. Stone quarries have a white-like face in rock, and it's easy to spot the color difference. Clay is found on the ground at red mounds. Again, easy to spot. These are unlimited so you can actually mine stone and sell it if you wish for extra cash. You can also earn gold chopping wood while sawing your own logs. I chopped firewood, and this more than paid for the iron ore I needed (3 gold each).

BE SURE TO SAVE YOUR GAME PRIOR TO BUILDING YOUR HOUSE! You can not undo what you've built, so if you don't like a room you added, you'll need to reload. I saved after every change to my house, just in case I didn't like the options, either.

This^^

I didn't really mind the cost. I furnished all three houses manually. You might want to start collecting iron ingots and Corundum ingots for your materials though. The problem I had was the availability of goat horns. Buy them while you can and buy as many as you can. I went through a lot of Iron ingots so my advice is to purchase them when you can and never walk past on ore vein.

With the addition of Hearthfire, goats now drop goat horns as well as goat hides and leg of goat. So I would recommend a trip to the Reach where goats are abundant. There seems to be a 50/50 chance that a goat will drop horns. So for every 20 you slaughter, you should get 10 horns.

I built all 3 houses and fully upgraded for 50k I bought everything except the basemant which you have to furnish on your own