The firm I work for prices work by firstly estimating the amount of time that each team member will spend on the project, and then multiplying it by their hourly rate. We do this in fine detail, right down to the last 15 minutes per each activity. Then we pick an arbitrary figure that we think the client will accept, and put that in the proposal. As the assignment progresses, we record all of the hours we work and then we write off 80% of them so that we only invoice the client for the amount written in the proposal.