Kane Mar has written a nice article about fixed price projects and Scrum.
Beware fixed-price, fixed-scope contracts . These are the most difficult to navigate simply because if we fix both the price and scope then the only remaining variable is software quality. Scrum teams fix quality by having a robust (and invariant) Definition of Done. By fixing all three constraints; price, scope and quality we have a recipe for a death march  project.
Part of the problem is that we have educated customers to expect fixed-price, fixed-scope by our past behavior.