For this type of work, the payment amount does not depend on the volume of resources or time-spent on the project. Fixed price projects may take a little longer to start, however, time until completion is usually quicker than anticipated.
Fixed price is a popular option, especially in cases when you have strict budgeting but extensive functionality is needed. The process includes:
Works outside of the signed specifications can certinaly be completed, but for an additional cost, and typically after completion of original scope of the project.