To be honest that is a pretty standard problem I encountered when I actually did freelance work. I think the best way around it is to:

A. Outline exactly what is included in your quote. Make sure they give you as much details as possible and explain anything left out of the brief will come at extra cost.

B. Charge per hour. Simple solution to get paid for the amount of actual work you do.

You definitely have to be firm and make sure they understand at the start so as to avoid an upset client when you refuse to add/change elements of the work. This problem, combined with clients who don't understand what they even want was the main reason for me not continuing freelance work.