Most of my firm’s contracts have terms which allow for flexibility (I’m fairly lax in my freelance stuff…they’re usually very small projects). The stated quote is a guideline of cost. It’s not a complete bill of what it will cost. We tend to bill a flat rate up till a certain point and then do hourly after that. So, for example, we might quote $X for a certain portion of the project up till 10 hours of work. If it takes more then 10 hours, we charge extra.
We always pad the hourly estimates so we know we can 100% finish within that time. If for some reason, we cannot, we usually don’t charge. But if the client decides to ask for a feature not previously discussed, we charge extra.
I think it’s important to discuss those things at the beginning of the project. We always make it very clear to the client that the statement of work is the primary documented related to scope of the project. If it’s not listed there, it’ll cost extra to implement.
The number of revisions are also strictly enumerated in the contracts. We go to great lengths in meeting with clients, discussing what they’re looking for, so usually we don’t need more than 2 or 3 designs. If you are someone who doesn’t do go to through the process of long discussions with clients, it might be more appropriate to supply a greater number of designs.
I think the key is not necessarily to anticipate the changes to the initial scope that are going to happen. I think that’s a futile exercise. More important is to have procedures and contingencies when those changes occur. And the client should be made aware early on what these procedures and policies are.