We purchased some work fairly recently. It had a turnover of about £26,500 per year and we paid £22k for it. It is a lot of money to pay, however the guy I bought it from had been cleaning it for just over 5 years and the work is all very close and he had refined it so well that it was worth the risk, and it is a risk because £22k can go towards a deposit for a house. The whole lot can be done in about 6 days by one person. Also there's a lot of extras we do such as gutter clearing, fascias soffits etc and pressure washing.
So it really depends on how the business has been run, how long its been cleaned for and how good the actual work is, money wise. Speculate to accumulate.