It looks like no Kindle/Nook-like device is going to be any use for sheet music, and iPads are too expensive.

Could a digital photo frame work as a music stand? They're big and cheap enough. But I've never used one.

Do they allow you to page through the images fast enough to be useful, and then leave the image up as long as you want, while running on battery power for a reasonably long time?

Something that accepted memory cards would be ideal, so you could keep distinct collections of scores on different cards.