In most cases I will prepare either a pencil sketch or a quick computer mock-up and combine that with an estimate. I have a distinct advantage over my competition in that I am handy with a pencil and I have a deep portfolio. Usually, a new client comes to me as a referral or they have seen my past work.
In many cases I invoice the client for illustration and design (selling specific usage rights). They are welcome to take the art down the street and try to get a better deal on the sign. I outsource everything and only have a small shop to meet clients and drink a cup of coffee. Because I have such low general and administrative expenses, my prices for installed signs are lower than most of my competition (who have to maintain larger shops, employees, service fleet, printers, spray booths, etc.).