If anyone's curious, it's possible to get every piece of gold without upgrading your speed at all.
Oh, and the max levels for the upgrades are: speed: 20, Ollie height: 20, trick points: 30, gold earned: 50, extra time: 12
if the medal doesn't award, undo one upgrade, refresh, and add it again.
I found that the best grinding method is to just get the 25 and 100 on the first level and then immediately restart. By the time I upgraded "gold earned" completely, I was getting about 400 every few seconds.