I didn't get to chime in on the other posts... but I thought I had something of value to offer to Mosh.
Best advice I ever got from an old sign painter... "Price it till they holler, then back off a dollar"
Don't worry about Tom, Dick and Harry's sign operation's pricing structure... worry about your own.
You want to make real money in this business? Make real profits? Start producing a quality product in a timely fashion. Actually fill out the entire shop overhead form in Sign Craft's price guide... and set your pricing structure according to what your shop is capable of making per hour.
When you say you letter a truck for $75? You've just lost money... big time.
Second best piece of business advice I ever got... from the moment you wake, everything you do or don't do has an hourly rate attached to it. As a business owner, you can potentially make money every hour of the day. So, when you are lettering a truck that takes two hours, (Realistically now, from computer time to clean-up) You shop should be making at least $60 - $120 per hour. That number obviously depends on your true hourly rate, as discussed above.
Let's say your shop rate comes in at $80 per hour... that truck should have been a minimum $160, just for being in your shop. (Do you have liability insurance to have vehicles in your shop? That's an expense that needs to be factored in as well) Now, you still get to charge a minimum of an hour for computer design time... another $80. Plus materials with an appropriate mark-up, say, $4 per sq. ft. if it's cut vinyl, and $8 if it's printed. Don't know how large of an area you consider "Lettering a Truck" is, but if it's just the doors, and nothing else, your between $48 to $96 depending on how you did it. Then you actually still could, if you wanted to... charge an install fee to cover your worker's labor costs as well. Say, half of the shop rate at $40 for install. (Your worker should make between $10 - $15 per hour)
Let's add that up, shall we...
I'm getting $376 on a pair of doors, digitally printed. That may or may not be too high for your region, but this at least gives you an idea of what you "could" make if you really had a product "worth" $376.
Now you have room to allow the customer to negotiate a discount... and you can actually honor that request and still make money. Now if they have a fleet of trucks, you can actually work up a reasonable quote that's good for both parties.