Clean up negotiation

The kids have been playing in the basement for the last 3 days. I have been avoiding going down there to see the mess. Today, I asked the boys if there was a mess down there and they said yes. I told them that I was coming down to check in 10 minutes and that they should go clean up. Spencer starts down the stairs and turns to say, "Okay, we'll clean up, but you have to pay us for it." So I turned to him and said, "Fine, I will pay you for cleaning up the messes, but you have to pay me for every mess you make." He agreed and went down to clean up. They came up when they were tired and Spencer demanded his money. I said, "okay, get your money box." They both ran to find their money and came back. I told Spencer that he needed to pay me for making the mess first. It was a $5 mess. He paid me $5. I then said, "well, you didn't clean up EVERYTHING, so you did a $3 clean up job. He seemed disappointed that I didn't pay him $4, but took the money. Carter paid me $2 and I paid him $1 for his work.
Overall, I got $3 and they did some work without crying. The rule is, if you clean up without getting paid, you don't get charged for the mess. Eventually, they will clean up everything and we will break even OR they will clean up without asking for money, either way I win!