I'd agree with Malc, if it was well established and everything about it stood up to careful inspection then it would surely be worth half it's annual turnover.
Were it a round just built up over the last few weeks via a roundbuilder then I'd personally be very wary of even paying 3 times it's monthly value.
Also, is it only 50k for some guy who works like a trojan and can knock out 10 houses an hour!!
I wouldn't want less than 50% of my round's annual turnover should I ever feel the need to sell it, but there are so many variables to consider; like how experienced a window cleaner is the buyer? Or, does he use WFP?
If it was a total newbie then he would probably lose a lot of the business through his inexperience, and it would take him several months to get up to a reasonable standard and speed level.
If a round of that value was offered to me, and I wanted to expand my business and it ticked all the boxes then I'd certainly consider paying out 25k for it, possibly along the lines of some form of monthly payment over an agreed period of time...
Ian