I'm glad I came across this thread. I have just been faced with this issue this week. I myself am still fairly new to the photography business, however my sessions are ever increasing in frequency. I had a client call from a session back in November. She paid the $30 session fee (which is another price I need to put some attention to) and then never ordered a single print. She called last week and requested a CD of all of the images bc she couldn't decide what she wanted, she just loved them all. I did a bit of research and ended up telling her that I would have to charge $200 for the disc of full res images. I wanted the price to be out of reach bc I don't want customers choosing this option, but I feel that I have to offer it to stay viable in the digital age. Suprisingly she was ok with it. The reason I don't want customers to choose this is for the same reason as a previous poster mentioned, they could take their images to Wal-Mart and get some crappy prints made and flash them around town saying they are my work.
I haven't really been presented with the single image request except for around Christmas when people wanted them for cards. At that point I sold them for $10 for the low res file (which is what everyone ordered). I think I will stick to the $10 for low res (4x6, 100 dpi) and maybe $20/$25 for a full res.
Also, one thing I have started doing is a day or two after a session, I will email the customer with 2 or 3 of my favorite shots, sized WAY down and watermarked. Kind of as a Sneak Peak. That way they can forward them along to family/friends/coworkers. It has been a big hit and has gotten me a number of referrals already. Just a thought.
Sorry for being so long! Thanks for bringing up this topic. 