Why not just charge by the hour period! An hour worked is an hour charged. If the client balks at that, get another client. That is how we do it. I've been burned on that fixed price garbage before and I swore I'd never do it again. If the client absolutely insist on a fixed price, make a very honest estimate of what you think it will take and then multiply it by 10.

So your $3,000 estimate will be $30,000 and honestly that is probably what it will take. Don't cheat yourself. You won't stay in business long that way.