Well not to be intentionally vague, but it really depends. Everyone asks a different price. It also depends on if we are talking a Fine Molds, ERTL or CC X-Wing. Do you have the kit already? Do you want lights etc......
I don't build for people anymore but when I used to I used to get $40 miniumum for painting statues only.
For build ups, I started at a flat $100 no matter what the build and went up from there.
45+ jobs from people and friends here on the RPF in 2-3 years, with many many repeat customers, I never ever had one single person balk at my prices and turn me down.
From what people told me I was cheap and could have gotten much more for my work.
With some asking around from the big model building guys on the net and showing them my work said they would have charged double for what I did, but I figured I would keep my prices low and get more work that way.
To this day I still get an occasional person asking me to build, but I just don't have time anymore.
It will usually depend on the builder. It depends on how much time they spend on building the kit (accurazing, getting rid of seams), and then the painting. I know some people who put alot of detail and time into it and charge a fairly hefty price for there build-ups.