agreed with @erictgrubaugh but only works best for both parties if the requirements are CLEARLY DOCUMENTED. or if the definition of change management is agreed upon by both parties. usually seen too many fixed bid projects where client wants more (but cant because department budget only approved the orignal SOW and they will need to get a change order signed and its a process) or the consultant is ready to jump elsewhere because you are now asking 200% of the SOW an they have another project to start. Fixed bid is the best method but REQUIREMENTS MUST BE DOCUMENTED THOROUGHLY.