I would never bid a fixed amount with a client like this. The only way to do a fixed-bid is if a) the requirements are 100% defined, b) appropriate controls are

I would never bid a fixed amount with a client like this. The only way to do a fixed-bid is if a) the requirements are 100% defined, b) appropriate controls are in place to account for scope changes and roadblocks, and c) the price of the overage risk were priced into the bid.