Depends what you’re buying, my general rule of thumb is x5 for decent WFP work. It’s also worth remembering that if it’s only part of a round that’s being sold, it’s generally the worst paid work that the seller will be getting rid of, so it’s often better to buy a round in it’s entirety. I don’t even entertain buying trad work now, it can be difficult to convert customers without losing work, every house can be like a first clean as top frames are usually dirty, then the customers that do keep you on feel hard done by when you clean their house in half the time of the previous cleaner.