Aside from the suggestions below, I don't think the money ball should have a damage upgrade. It should be speed or a damage reduction instead.
The purpose of the game is to break blocks and generally the faster you break blocks, the faster you can make money. That money helps you break more blocks faster and the progression continues...
But the purpose of the money ball is to make money and because money is only given when they hit blocks, they work best when there are many blocks they can bounce between. The ideal scenario is having a money ball start in the middle of a clump of blocks right at the start of a level. They can accumulate a significant amount of money this way because they bounce so many times in a very short period of time.
Having higher damage reduces the effectiveness of this strategy because they'll break free faster. So as a player I have no incentive to purchase the damage upgrade because it nerfs how much money I'm making.
I would even prefer to pay for an upgrade that causes them to ADD hits to a block rather than remove them.