I asked my son about that and he said $100-$150 depending on the pic selected. He said that the light on the car in the pic can make a huge difference in detail. I'm not trying to run a business here, but if somebody here wants a damn good drawing PM me.
You should have a pic that is free from reflections from trees and things. It should be exactly how you want it to look and be a file that is large. Like my drawing is 8-1/2'' by 11''. I would suggest a handfull of pics to choose from. Make sure the file is at least 1 MB in size to make sure they can see it well.