You have the right idea with Smithing. It's XP gains are based off of item value. The trick is... to get the most value from the least amount of material used. In the early levels, one must also consider the added base of 25 to every item crafted. That means that an Iron dagger , while noted as being worth 10 gold, will yield the XP from a value of 35. Compared to Iron Armor with 5 ingots, the XP gained from daggers will net 175 as opposed to the armor of 125.
And the Dwarven Bows are the next targeted item when you get the perk. It will take you to Ebony, where you can start crafting Ebony Bows. This is where the threshold is crossed to where you will benefit on a per Ingot to XP gain by improving the Ebony Bows as well. that will rocket you to 100 Smithing.
Enchanting is based off off a flat rate XP gain per item. no matter the enchantment, value of the item or the size of the gem being used. It's all the same. The best rule of thumb for Soul gem usage is...
Petty - Enchanting XP gain
Lesser - Enchanting XP gain
Common - Enchanting XP gain
Greater - Weapon / Stave recharging
Grand / Black - Enchanting gear you actually use