All software development contracts I've seen specify that x piece of software will be developed for y dollars within z amount of time. This works out for traditional Waterfall model projects, where you do a lot of forward planning and it is reasonable to expect that you will know how long the project will take near the beginning of the project. It doesn't match up at all for Agile, where you can start without having any idea exactly where you will end up and you are expected to be able to deal with requirements changes.
How would a contract for Agile software development differ from a contract for Waterfall software development? Is there a place where I can find a sample contract for Agile development?