I really enjoyed playing through this, and completed it in about an hour. However, I agree with many of the other comments. Three small suggestions:
1. It would be nice if the maxed out upgrades changed color to signify that they can't level up anymore.
2. Perhaps change the upgrade cost scaling or the appearance frequency for the dollar bills. Once you hit a certain point, the dollar bills are worth far more than the trash, and you can complete the whole game pretty quickly by only farming dollar bills.
3. Since I was playing on a MacBook with high resolution and a relatively small screen, the UI scaling was a little rough on my eyes.