Sometimes, non-technical people don't (or can't) tell the difference between a "bug" and a "change". when the thing doesn't do what they expected in their mind's eye, its a bug, not a change. A change is when a completely new feature that isn't mentioned in the spec.

So "bugs" like, moving a menu around, or swapping screens seems easy to someone who has never written anything before, but it isn't trivial (unless, of course, you had time to write nicely abstracted code. But for $800 and two weeks, i doubt that's the case).

May be there should be an app that lets clients make mockups, and when they change their mockups, it computes a "diff" that then is heuristically used to generate an expected price...