I used to run a small business, selling and repairing PCs. I had Those Customers every once in a while: everything's a bargain, "is that your best price? are you sure? I can get it cheaper at/from (some big box store / some shop in Halifax / one of my competitors / my brother in law / mail-order from Toronto) you know." Or, "Is that your best computer?" (What kind of a question is that? Best for what?)
Seth Godin talks about something similar. His post made me think of one of my favourite stories from Zen Flesh, Zen Bones:

When Banzan was walking through a market he overheard a conversation between a butcher and his customer.
"Give me the best piece of meat you have," said the customer.
"Everything in my shop is the best," replied the butcher. "You cannot find here any piece of meat that is not the best."
At these words Banzan became enlightened.

(No, not really related to Seth's post; as I said, it just reminded me of it.)
I didn't reserve my best computers for my best customers, or my best work either; now I work equally hard for every faculty member and grad student, regardless of how I feel about them personally. You will find no work that is not my best.