I think I'm just gonna have to disagree on this one, and point out that, it is completely incongruous with other existing game elements. Like you, I have noticed that my sneaking seems very effective in PA, even without a stealth boy.
Consider that, bulkier armors in general are purported to reduce sneakiness, and there are various mods for standard layer armor pieces that are meant to buff them and reduce this negative effect on sneaking. Given this, and the fact you can HEAR the damn suit clunking and churning as you walk, it just does not make sense to me at all that there is ANY option to sneak in Power Armor (or to get the stability/accuracy boost from crouching either) short of flicking on a Stealth Boy. The suit is enormous, it must weight 2000 if not 3000 pounds, it makes noise and unless the mechanisms that translate your movements into the movements of the suit are extremely advanced it should largely negate one's agility.
I do hope to make mods that are popular and exemplify good design though, it isn't just "for me." One aspect of good game design when it comes to controversial points or ones where there are differing opinions is to include "options." So as long as I can figure out how to do it (which presumably will just be a matter of time spent trying to figure it out) I am happy to include an "options" pane where players can turn on/off such things 
Good to know this is one thing for which an on/off toggle might be appreciated.
Ah good feedback, thanks! Okay, maybe a graduated scale of repair cost with T-45 having the lost multiplier and the X-01 having the highest. For the Durability, it just seemed 'reasonable' to up it at least a bit given I'm talking about rather large increasing in building costs and other constraint. Still My inclination is to start out at about 300% repair costs for the lowest tier T-45 and to go up from there. The goal I'm striving for here is to make Power Armor feel "Special" not just run of the mill.
The goal would be to make "wearing power armor" less of a general "anyone can do this, as long as you find enough fusion cores" option and more something you really have to build a playthrough around, while simultaneously providing enough benefits that it remains a very worthwhile strategy to take. Basically make it less appealing to a player build in general but even more appealing for a specialist build.
It is a good point about the frames being abundant, but really what I was thinking is that: it just doesn't make _sense_ that I can magically make a machine gun or laser turret out of spare parts and junk, but I cannot loot and scrap power armor frames. Eventually I hope to have a mod in which virtually anything you use has "degradation" and requires upkeep (weapons, clothing, armor) so making PA frames degrade too would be part of that. Again, an "options" category with several choices: "Vanilla/ Only Weapons / Only Normal Armor / Weapons & Armor / Weapons, Armor and PA Frames / Everything" seems like a good plan.
However the thing here will be: if Power Armor pieces and frames _are_ scrappable at the bench (which I would intend to make them), then making your PA Frame "not" degradable might lead to excesses of materials . . . Within reason, I'm happy to try to include some simple on/off switches for options, but with the caveat that "the mod overall was not designed to support all settings equally."
As far as "how" you would scrap them (given I'd also like to make all weights and material values more closely in line with reality) . . . I am guessing I will have to incorporate something like a courier system that builds off of one's allied settlements. I don't recall much about that from FONV but something along those lines presumably. So, "looting" a PA Frame would ideally NOT be so simple as: you see the PA Frame item in the dead person's inventory, you click it and you get a weightless items in your inventory equivalent to the "Power Armor Frame shipment" you can buy from vendors. Rather it would be a matter of arranging for a courier or something to be with you and you assign them to tote it to an allied Power Armor bench. Related to this, making any Power Armor Frame that is in range "collapsible into" the work bench might be a nice feature, and then the ability to scrap the frame once it is in there.
Definitely good input. I think it would need testing to decide what was the best number to plug in, but I'd like to error on making it truly "prohibitive" to imagine for anyone except the sole survivor or the BoS, or the occasional raider lord to be able to afford it. 5000% might be too high, but my initial reaction is 300% is too low. The ideal would be to make it _TRULY_ constraining to use the stuff, but also truly _POWERFUL_ a true game changer, albeit at a cost that is also a "game changer" in the opposite direction.
At present, Power Armor is powerful (maybe not as powerful as it should be but definitely powerful), far too versatile, far too cheap and easy to use. My goal would be to make it much MORE powerful (in those dimensions where it really should shine: tanking, environmental protection, and firepower) while also making it much more constraining and costly (NO sneaking else VERY expensive Stealth Boy use; no aimed shooting without a very expensive HIC mod, etc; extremely expensive fusion cores, etc.)
Well, I'm not talking about making it _less_ possible to use heavy weapons without power armor (which arguably _should_ be the case at least for the mini-gun) but rather to make it "more" beneficial to use them while wearing PA than not. Heavy weapons are, well HEAVY! The produce more recoil and in the case of a mini-gun, possibly a lot of angular momentum which the user would experience as twisting or jerking. The game mechanics basing Heavy Weapon use on STR acknowledge this, and my intent is to elaborate on this existing game "design logic" by accentuating the benefits of using heavy weapons while wearing PA.
I look at this way: if you are wearing something, that somehow boosts your STR and END or your INT and CHA or offers you protection, it should very likely continue to do so just as effectively when you get into a car, or mount a motorcyle or otherwise enter any vehicle. If you are going to be shot at, would you rather be naked inside an armored car, or would you rather be fully armored inside an armored car?
Nothing about being inside an armored vehicle (even one that was tailored to fit closely around the human body) should "negate" the beneficial effects of clothing (or performance enhancing drugs, etc.) in real-life and apart from "game balance" it is difficult to discern any legitimate reason for doing so in the Fallout game world.
Perhaps it will make PA totally OP, but my gut instinct is to make it realistic, as realistic as possible, and then tailor the other constraints, costs and tradeoffs as a way to balance, instead of applying a magical NERF to gear worn while inside a suit of Power Armor. With these comments in mind, perhaps it might make more sense why I'd be inclined to make fusion cores extremely expensive and PA maintenance also very expensive?