Most past contracts for software development are based on an assumption that it is practical to specify the project comprehensively before the major cost is incurred and that this specification can be costed accurately and the project delivered to a fixed deadline.
We know that 70% of these contracts result in legal costs being incurred in a dispute over the contract deliverables/payment (UK figure).
So we must seek an alternative approach.
The major alternative approach is called an Optional Scope Contract.
Kent Beck and Dave Cleal have written a paper on this subject published in PDF at: http://www.xprogramming.com/ftp/Optional+scope+contracts.pdf
I cannot add much to this excellent document but I will take the ideas to a New Zealand solicitor and try to get some relevant wordings.
The hard part is convincing your customer that the XP approach is not just a refusal to commit to a fixed price.
I would welcome any approaches from the field. Use the Comment link below please.