you need to be a bit clever with them
search the internet and find some prices pages of other pressure washing companies and print them out,
tell your friends you are having a problem sorting out your prices for the coming year, you have checked other companies prices and you are wondering if they think the prices quoted by other companies is reasonable ( then show them the printed out prices)
say of course you would'nt charge these prices to them......but it would help if they gave their opinion on what other people are charging.
this stratagy will subtly tell them how much the job should really cost them, so if they say the price are reasonable then say you will knock £100-£150 off because they have been such a good help in the past.