The pricing of a project depends on the complexity of the work and this complexity is defined by your goals and needs. 
I work on a project on a fixed bid basis which means that there is a fixed, single price, for the whole project. It is decided prior to starting the project after clearly listing out the deliverables and timelines. To be very frank, urgency and competitive market pricing are also factors which influence the price. However, repeat business from same client makes the pricing more competitive as the relationship grows from project to project. 

The work normally starts with a 50% advance and a formal approval of the proposal. The balance 50% is paid by the client on project delivery.

In some case the project milestone may be more than two. The payment schedule for such an arrangement is mutually decided based on milestone definitions and work load for each milestone.

If, during a project, the deliverables get changed or there are changes after approvals at certain stages, all leading to increase in workload then it merits extra charges which are mutually decided.

For design firms and advertising agencies, who would like to hire me on project basis,
I offer a competitive freelance rate that is significantly more affordable! but that's not applicable for a one off project ;-)