Back in the day, we did art for free to sell the job. The customer wanted to see what his sign would look like before he plunked down his money. Design was an overhead expense. We made our money selling plastic, wood and metal. In time we accumulated a fleet of service vehicles, and installations and maintenance became our main source of income and profits.
These days, I make my money doing design work. There's dozens of sign shops around that can order signs from wholesalers and put them up. There's even a few who still fabricate signs, but the trend these days is outsourcing fabrication. I have a lot of respect and admiration for sign shops that make their own signs and maintain a service fleet, but running a profitable fabrication and/or installation business is a lot of work and requires substantial investments in facilities, equipment and employees. I prefer to keep good relations with local full- service companies and steer my clients to them for furnishing, installing, and maintaining signs. These companies, in-turn, send me design jobs that bog their production artists down.
There are full-service companies that do not like my business model. I can always just order the signs from the same places they do, and have them drop shipped to installation and service companies who are quite happy to provide those services and have neither the inclination nor the technical ability to design signs. I don't want to sound like a jerk, but I can usually come in with a substantially lower complete project price and still make a tidy profit. These same companies that I piss off seem to be happy receiving shipments from sign forwarding companies that work for national accounts. I don't see how what I am doing is any different. If the local full-service companies want to work with me, great! If they don't, well that's their business.
It's important to note that I offer consulting and engineering services in addition to graphic design. Many of my clients want me to estimate their entire project cost, and retain me in order to get competitive bids on completely spec'd projects. By offering a full range of design services, in most cases I take that burden off the fabrication and installation companies and can deliver a full set of drawings and specs that can easily be priced out. Most municipalities and many building owners will require comprehensive engineering drawings for larger projects. They want to know the pull and shear strength of the attachment hardware, how the weight and wind-loads transfer to the attaching structures, the section modulus of the pipe supports, and how deep the hole is and how much concrete needs to be used to safely ballast a pylon. It is also important to understand how commonly used materials are deployed in the construction of signs and how equipment is used for successful sign erection. At the end of the day, it is the fabrication and installation contractor that is responsible for the signs they erect, but it makes the bidding process much easier if they have a full set of plans to start with (I do carry errors and omissions insurance; the cost is low and I've never had a claim).
The process is the same whether I'm designing a banner or a billboard. It's just a matter of scale. The bottom line is that I charge for design, and that is a separate ticket from fabrication and installation. For smaller jobs, I take a deposit. Larger jobs require a contract with a designated payment schedule based on a time-table of deliverables.