How much?
#1
#2
How much?
Depends on how good the shop is and how many tries it takes them to get it right. It could take anywhere from 1 hour up to 3 hours if it gives them trouble. After paying a $180 bill for a rejet and valve adjustment I learned how to do it myself real quick. It's not that hard if you find a tutorial to follow. There's a good one with pictures on yfm350central.
#3
#4
How much?
it could take more than one try. Finding the right jet size and needle setting is not an exact science, so it usually takes trial-and-error to figure out the best one. Some people will stop trying as soon as they get it to the point that it runs ok, but the quad won't be running at it's peak power until it's jetted perfectly.