The "right" way of doing it would be to charge based on how much you download. This way if you don't care and only want 128k/s then you'd pay less than somebody who wanted a proper archive quality one done in FLAC. This way you're both paying a base price for the track and then whatever extra is the additional bandwidth you're using up by getting the bigger file.
The only problem with this is that the average person out there wouldn't be able to understand it easily. I know people that can't tell the difference between RAM and HD storage so don't say it won't happen.