Upgrading your mobs was fun, but I did feel like I often had basically unlimited gold (except a few times against the final boss). I like the idea of gold being based on picking up things on the track conceptually though - measuring how far your wave is getting and rewarding you for it makes sense.
Might be possible to solve the economy issues with a 'phase' system where you get multiple chances to upgrade / enemy to upgrade during a level, and you get to choose your upgrades based on what the enemy is building / upgrading. So there's a set number of flowers per phase, and it's a bit easier to tell whether you're doing well. Definitely a tricky concept to balance though!
The way the princess vibes out at the end of the track is very funny.