If it's a titchy little 3 bed semi that takes 20 minutes then I just charge what I charge regulars. If it takes 5 minutes longer because it's not been done for 10 years...so what? And if they've conned me and it is a one-off after all, again...so what?
Where I do slap money on is the bigger stuff where the extra work does use up a lot more time, labour and materials. I try to keep the quote a bit open and just give an approximate figure then charge my hourly rate.