I would never recommend going into a commercial job, without the budget or money figure the client has budgeted for his particular requirements, and I will never push the shutter button without knowing all figures in place before hand, once the job is done and you are there to deliver the product, IS NOT THE TIME TO NEGOTIATE! you must do this before the job to ensure payment and delivery, I always try to work with a contract in place, if a customer will not sign a contract, I really doubt how serious he/she is about actually getting the job done, working with a contract in place, with all facts and figures really can cut down any hassles that may arrise once you have shot.

As John mentioned, make sure you know the market in your particular area, this can vary based on location even within a single city, I always try to get the high and low range of what the type of work I am doing is going for, then price myself somewhere on the high middle range, based on my experiance and what I have done and can show the client in the particular field we are doing the shoot in.