I would say that if you pay commission then pay them a percentage the cleaning charge. Say around 10-15%, then they would make an effort to increase their earning, rather than going for low priced jobs they would try to get the best price possible for each job. Which is what would happen if you paid a fixed price per job. Maybe you could also offer a incentive as a bonus for any jobs of £20 and over, say 20%?
You would still have the ultimate say in the type of jobs you require, and the pricing structure. Anything that doesn't meet the specifications that you set out, you can refuse to pay, because it is not what you asked for.