Because I've started doing this more recently, I decided to start to catalog some things. I'm only going to be giving vague price ranges, nothing's absolute until I know exactly what I'm working with.
That being said, there are several different ways for me to fix areas. I have a plethora of scrap POM and Nylon pieces, and some can be modified to fit certain areas. While I make sure the setups are tough in general, you do lose some structural stability when I modify pieces from their original molds. That's because of A: centerline pororsity (thinned areas in the center of the mold due to the outer surface cooling more quicker), and B: voids (bubbles usually centered in the plastic, sometimes they're closer to the surface, and I can even hit them working on preexisting pieces sometimes).
There's also scratch built pieces, used from extruded POM, both generic and not. The plastic I'm using is consistent with, given all of the figures I've tried out, both the robot damashii 00 Seven Swords and Strike Gundam (I've only ever seen one report of one of these breaking, and it was because of the immense stress applied on the joint). It's extremely tough but given the right amount of stress, will not show much signs of breaking, and simply breaks. Which isn't to say it'll break easily... at all, but it really doesn't show much elongation at break. Now, that's only for my Delrin 150 E, which I have in Natural (off white) and black. The black appears a great deal weaker, kind of like the modern SHF Iron man and war machine, so I'll only use it if you absolutely want something to be black. I have a generic black POM that has a much greater elongation at break, and I think from this point forward I'll be using that more often than the delrin, OR more often for sockets rather than things which require a great deal more toughness, rather than elongation. The generic black POM feels kind of like older SHF figures or some D-Arts figures. Contrary to what it may seem, a lot of D-Arts plastic isn't breaking because of its strength, but the setup.. which is usually atrocious. The joint sections are too thin, too tight, etc.
-Using only the original pieces; gluing and buffering with metals or scrap
-Scrap POM/nylon, extremely modified, from gluing sections on to others to ignoring voids almost completely.
-Scrap POM/nylon, very little modification
-Scrap and scratch built. Probably the most efficient option, less pricey and doesn't take as much time.
-Scratch built, no centerline porosity or voids, more efficiency for specific part width and etc, more expensive. Very little choices in color, only black and white, I would suggest always using the white as it's not hindered by the dye.
All of these choices can be applied to original pieces if you wish that to be, the first one just excludes all of the ones below.
Similarly for all choices, I will make the attempt to paint areas which show up on the final product, or hide them with PVC or ABS covers. POM and Nylon cannot bond well to most things, as such it's very difficult to paint without it easily expelling them from rubbing alone. However, that being said, most setups will have to be attached with glues. For example, attaching a double sided ball bar to the crotch area. I usually include rivets so once the glue has dried, it won't break the bond, and even if it did, it won't remove the piece (I've never had a glue bond on POM break in areas like this, only when I glue straight POM on POM does it tend to break easily.
You also have the choice of allowing me to reengineer the sections that were one broke. For example, instead of relying on pulldown hips on a certain figure, I can either A: do a normal hip setup without the excess articulation, if you don't really care, or B: set it up so that the sockets are closer towards the front, and lower, so they allow for a similar amount of articulation. The problem is usually that it's NOT as articulated, but sometimes more so. The ultimate benefit is strength, since you have a single center rod instead of 3 pieces combined together. Keep in mind that even if you just want a replacement piece for, let's say, and SHF hip replacement, I still tend to make them larger so they're less likely to snap.
A recent project, while still getting my barrings setup, was $27 for an ass plate addition and hip replacement on D-Arts Knight Blazer. After the work, thought it was hard, I realized I had done a lot less scratch work, so I covered shipping (the initial price wasn't ever intended to include shipping).
Everything said, every situation will be discussed a great deal before actually getting started.
I also prefer doing things in bulk, so please please be sure to get everything you want fixed altogether, because you'll also get a minor price decrease with this.