in your case the customer has been honest all one offs are 2.5 - 3x the defualt price so about £30 is fair. but read below if you,re canvassing to find out whos a one off and whos not.
new customers should be always offered a one off clean before you quote your defualt clean price , its not 100% full prove but by saying "was it a one off"? the very first answer they say is normally what they will be after the first clean.
if they say yes a one off pls then maybe reg, its 2.5-3x the defualt price if they are quite satisfied then you tell them the next clean price and give them x amount credit that they get back over say 6 cleans.in my case about 1 clean value.
but dont fall into the trap of "oh , actually can i have a reg insted" thats only an attempt to get a one off for the price of your defuat price u must go with his or hers first instinct and stick to your guns!