Charges for tire changing are all over the map. Some shops throw it in for free if you buy tires from them (but may charge higher for the tires to cover), some charge a nominal amount (less than $20/wheel) to attract business, others charge at their standard labor rate. Some refuse to mount tires purchased elsewhere, others do it grudgingly (and charge exorbitantly and/or throw in making you feel like a cheap-skate 2nd-class citizen for free), others do it cheaply to encourage you to consider buying tires from them in the future. No hard/fast rules on this one.
If the tires are competitively-priced and do not include a markup for changing, I see nothing wrong with charging $35 or so per wheel (if you bring the wheels in) as it does take some time, especially if they do a good job of balancing. If you're having them remove the wheels from the bike, it can be a somewhat more, especially for some bikes that have a complicated wheel-removal process (the S10 is reasonably straightforward).
I do my own, although as I get older, this sort of physical cave-man mechanics becomes more difficult. But the DIY savings are pretty significant. A fresh set of mail-order tires shipped to one's door that cost $250 and can be mounted/balanced for free could easily cost $500 all up at the dealer and the dealer's stock is often quite stale. There are some initial costs of getting setup for DIY tire changing, but you can make it pretty cheap if you're creative. Or you can spend a fortune if you want a top-end tire-changing machine like the dealers use. Or somewhere in-between (e.g., Harbor Freight, No-Mar, etc.).
This has been the bible on cheap DIY home-brew motorcycle tire changing for many years:
http://www.clarity.net/adam/tire-changing.html
- Mark