Always a sensitive issue in my mind. Still shocked with the $35 TM stock spring guide and $55 metal one. If the orignal one was broken, a good doctor would have told you to go right for the bearing spring guide, who to contact to buy one and the price (generally $25). Charging you for the price of the TM spring guide (man, not sure if he buys the parts and charges labour on top) even after it broke........... not sure. Been times something has broken I've worked on, I've either fixed it, replaced it, or the owner decided an upgraded replacement part was best for them to buy. I always go by letting the owner buy the parts they want, sometimes I recommend what to buy (times some want every part upgraded, brace themselves for the cash it'll cost, only to find out that my recommendations of good metal bushings, spring and bearing spring guide costs the least and will be reliable for a very long time).
On the other end, if I charge someone to work on their gun, and the gun screws up after it's left my hands (Shagg's M4 with Guarder FTK comes to mind, 6 locaps through it no problems, he gets it, fires 2 shots and the piston jams) then I keep fixing it til it's perfect. Luckily once a mechbox settles in after it leaves my hands, its good for a long while.