I find it tough getting good prices for add-on work, unless it's a real bugger of a job. The thing is, I have these regular customers, and I'm fairly happy about all my prices for window cleaning, so then I don't want to put a spanner in the works by charging high for one-off type additional work like conservatory roof cleaning or gutter cleaning.
I have in the past, annoyed a customer who I subsequently lost, who thought I was being greedy when I charged her what I thought was a fair (but verging on the side of high) price for a gutter cleaning job.
I still charge a fair price, but I don't kick the bottom out of it, and I often qualify it with, "And that'll be a lot cheaper than if you got a quote from out of the Yellow Pages, and if you don't believe me, you're welcome to try it!"