There are two types of songs I'd pay $3 for.
1) The type of song so universally loved that every guest in my house would bash me for not owning it. That 90% of XBox Live players will own at any price. Something like "Sweet Child of Mine" that is universally popular across sexes (note this is not even close to my personal GnR song and I would expect it on the actual game disc because it is so huge).
2) An essential genre title far enough out of the mainstream that it wouldn't attract a lot of buyers even at a low price point but is essential for genre fans. Like "Somebody's Gonna Get Their Head Kicked in Tonight" by the Rezillos (which is actually a cover so ineligible for RBN).
Basically it has to be something that will get played a ton, one way or the other. Which is why I will personally rule out a 10 minute magnum opus because odds are I'd only play it a few times and rarely at parties.