That depends, if its half a dozen on a house you might get that, Id have thought thats a bit on the high side to be honest though. Any house with a half decent set on are going to have 10 or so, thats fifty quid and theyre not going to take more than 10 minutes. Lets face it, you dont need to rinse them in the same way as you need to with windows being sure that the glass is clear.
Ive got a job with 240 on, if they were £5.00 a pop Id happy as Larry.