I think that people are used to buying digital downloads of music and if the music helps them enjoy Rock Band or Guitar Hero more, more power to them.I think having full fledged releases on PSN between 20-30 usd is OK.
It's funny that people complain for paying 20-30 US for a full game but will buy Guitar Hero or Rock Band tracks for 2-3 US.
There are times I question if it was a good idea trading in Rock Band from my PSP, perhaps it would have been better to give it a whirl and see if I liked it, after all branching out and growing is important for gamers, and is it really a good idea to keep buying the same game (i.e., Neo Geo games) for every system (PS2/Wii/PSP) that I own?
As far as pricing, if I can get the UMD version at the same price as the download, I'll buy the UMD version. I prefer hard copies of games over virtual versions. I did pay $23.99 for Left 4 Dead, but it was a rare decision.