Awhile back, I was speaking to a handyman friend of mine regarding how he and his peers set the prices for their services when a potential customer comes calling. He told me that most he knew have this rule of thumb:
(paraphrasing) "If you are really busy, shoot the person a really high number."
This fellow understands the notion of opportunity cost. When he's busy, additional work takes away his leisure time, time which is becoming more and more scarce and, as a result becoming more and more valuable.