I once had what I thought was going to be a pain in the bottom customer; she'd sacked her previous window cleaner because she didn't like them; she sounded like she was going to be trouble, so I quoted her £25 (her previous window cleaners charged £10).
She turned out to be an excellent customer; she even 'canvassed' some other jobs in her rural area which led to a lot more work, and I really felt quite guilty about taking £25 off her for what is really a quick job - easy access - so I knocked a tenner off it.