Always a tough call... one I hate making, as it's usually friends that want a copy. I charge between £10 & £30, depending on size and quality required, I would charge more for poster prints, or canvas, but in the end it's whatever you feel comfortable with, and possibly it could depend on what your customers can pay. I know that there are a lot more commercially savvy people on this site than me, and I am sure that they will have a different take on it
If you are making a living from this then it all changes.
LOL I know... no help at all, am I? Good luck with the sales, though. Nice to have a level of interest.