I don't agree with all of Max's negative points about paying hourly but I do agree with all of his points about paying per project.
From my own experience of having a consulting business and having to bid for projects at a fixed cost you do look to optimize in these areas in order to get new projects and make money from them:
1. Getting the project completed as quickly as possible
2. Making the software as error-free as possible, so they can move on to the next project and do not have to always support old projects
3. Reusing as much code as they possibly can, so the project gets done quickly
4. Stick within their expertise area, so they can accurately gauge quoted project price