well I guess it all just falls down to how why and where your doing the work, I mainly only mod consoles for friends and family at this point, so it really is just a few quid for time/parts, I guess if your selling it as a service $10 is a bit on the low side, I know at one point when I was doing hardmods I was doing them low at first, but there was too much demand and not enough time in the day, so I ended up bumping the price up twice just to better control the demand, once another modder in the UK jumped onboard and supply eased up I dropped the price back down a bit, this was all before NTRboothax was a thing, so I guess as with most things the price is set by the market really, if you do it too cheaply you end up getting to swamped in consoles to mod and mistakes get made if your not careful, too high and you won't get any customers at all, just play about with your price to find the optimal price point
As a little tip for anyone doing consoles at that point I used to make a nand backup from peoples consoles and archived them, then deleted them after about 6 months if I didn't hear anything back from them again, I did include a backup on their SD too, but even then I had about 4 people manage to screw up their consoles and accidentally update their sysnand or brick their console, but again at this point thanks to NTRboothax its pretty much impossible to irrecoverably brick a 3DS....the switch on the other hand if you mod those it would probably be an idea to keep a backup of the PRODINFO along with the console serial number for safe keeping