Theres some more good Mp3 debate in here, but alot of it is pretty off topic, not that I care, I just thought I'd try to actually answer the question, so here goes:

"my theoretical online mp3 store"

I reckon 99p a tune would be a good target, but I think being a bit creative with that is the way forward. Like maybe if you buy one tune, then the price is £1.99, but if you buy some of the remixes or what would have been the Bside, say a total of 4 tunes, you get them all for 99p each. I think that is pretty fair, if you really only want the one tune, 1.99 isnt so much of a rip off, but of course there is that encouragement to buy a few more tunes and get 50% off. And you'd have to do it so that you could take any four tunes rather than just the ones associated with a certain release, cause that ability to pick and choose is what will get people interested. It would be sort of like creating a virtual EP or compilation album or something, but at half the price.

And the idea of sending zips containing artwork and information is a must, every possible bit of value that can be added should be. Perhaps it would be an idea to also include some chopped up loops/samples from the tune thrown in for free, and then all the ableton bods would be pretty chuffed.

If there was a site that operated in such a manner, with a good selection of new stuff and choice back cat classics, I would definately frequent it, and no doubt spend alot of money on it. And if I had more time, or could be bothered to actually try and achieve something, I would try and set it up myself, cause I reckon it would be a money earner.