If you don't use your lawnmowers during a battle, you will be rewarded $50 for each one that is left. If you saved all five lawnmowers, you will be rewarded $250. Even though the lawnmowers can be used as a safety net, to run over zombies that manage to get to the end of that lane, you are also forfeiting $50 for every lawnmower used to kill enemies. So think ahead and see if you need more money to make purchases. If you do, then try to build a better army against the zombies, and don't rely on your lawnmowers to take care of them.
When you are willing to make money by working through all these activities, you should be a rich man and a busy customer at Crazy Dave's shop. Remember, most of what Crazy Dave sells runs into many thousands of dollars, so it won't be easy to buy his items if you don't make an effort to earn money.