I really think they need to do things differently from the 360 version for the downloadable songs. Maybe something like this...
You can download the songs as a single track OR in a Set a la the previous games.
Singles could sell for 100 points. This is roughly comparable to the iTunes Music Store price. This might be scaled up to 150 points, but 200 or more and I wouldn't use it.
Sets of 5 songs could sell for 400-500 points (possibly a small discount for getting it as a set). Assuming the sets are five songs, I'd be willing to go up to 600-750 points, but would really hope that they aren't THAT lame at pricing these.
Bulk purchases of singles might also allow for dicounted prices. Perhaps if you buy 5-9 songs you get 5% off, 10-14 or 10-19 for 10% off, and 15 or 20+ for 15% off. Keep in mind these are fairly small discounts ($0.05 - $0.15 per track), and could only apply for buying single tracks, not for buying them in a set.
500 points for 3 songs would be pushing me, just as 200 points for one would. I might get packs at 500/3, but I'd need to really like the songs in those packs.