most, if not all, pro's would never sell the original. Many people want a digital file for a screen saver or for something like MySpace posting. You only need a lo-res file for these purposes which in turn limits sizes that could be printed.
I've sold these for $5 -$10 depending on end user/.customer. I'd max out download size to that appropriate for an 8x10, add up you sell prices for 8x10's and smaller prints to come up with a download price for this higher res file. if they want a larger print, buy from your site.