I've 5-starred half the songs on the Medium tour and beat my first song on Hard!
On medium, I noticed that if you match the last attempt (star-wise), you don't get any money. You have to get the next level or higher. Does this happen at every difficulty?
basically it works out like this, when you beat any song on the main tiers from medium up through expert you get cash. each song has a set amount of cash you can earn for it on a difficulty. you can either get all the cash for the song by just five starring it or progressing and getting the cash in increments by 3 starring, 4 starring, then 5 starring.
Example:
song A yields a total of $600 dollars on one difficulty
3 starring first try = $100
4 starring after 3 starring = $200
5 starring after 3 and 4 starring = $300
4 starring first = $300 (sum of 3 starring and 4 starring)
5 starring after only 3 starring = $500 (sum of 4 and 5 starring)
5 starring first try = $600
i hope that makes sense