Whatever works for you. There is validity to your method but I guess it depends on your typical client base and their typical budget. If I were to charge like that I'd never get hired. . .I just don't have folks with the kind of money to charge like I'm always bringing all my audio gear, lighting, etc. etc. with me if I'm not using it. As to your scenario, if it's something complex enough to worry about, I try to ask all the questions I can think of, and charge for what I think I'll need (or potentially need) based on their answers. I don't have "production vehicle," per se' it's just my regular SUV, so it gets loaded/unloaded for each gig, so I can't simply "go to the car" for lights unless those lights were already there, which they wouldn't be unless I brought them/charged for them for that gig already.