If it is well established, compact enough, e.t.c. then i do'nt think the price is unreasonable, x3- x4 is an accepted industry price, when figures of x5 or higher are bandied about, you need to ask why its higher than the usual rate, it might be worth paying x5 if the outgoing window cleaner had streets where just about everyone on the street was a customer.
Also it Says £680 per month, but if they are all £4 per customer on average, which i have seen frequently up north, its a lot of work for not a lot of money!
However, its advertised as 3 days work, so its about £100 per day for each of the 2 man team, which is'nt too bad!
Paul