Like the poster above said, it's completely up to the developer. It depends on how much time and work it would take you to complete a Flash site versus an HTML site. There isn't a stock response to this.
I agree with the two previous posters. Currently I do both flash and HTML sites. I have base prices which are more of a guideline; however, what I charge depends on what the client needs. Having me build a total flash website will cost significantly more than a HTML site because I can work faster with HTML. For this reason I use HTML and add flash apps (intros, galleries, active media) to keep the cost down and still produce innovative, high end web sites.