IIRC, completing a song in career mode gets you $100 for a 3-star performance, $350 for a 4-star, and $750 for a 5-star (not counting the "bonus" cash for completing a category). So if you do a 3-star performance you'll get $100. If you retry it and get a 4-star, you'll get $250: the $350 for a 4-star minus the $100 you already got for the 3-star. If you then practice enough to get a 5-star result, you'll earn an additional $400.
The numbers might be off, but the concept would be the same. In order to buy everything in the shop (all the songs, all the characters, all the guitars and all the skins) requires money equal to getting all 5 stars for the non-purchasable songs (you don't get any money for the purchased tracks).
If you get 5-star on all the songs in the career track and 5 stars on all the purchasable songs then you get the title "Guitar God" for that difficulty level.