Didn't they just customize cars already? Also when I mean functioning vehicles, I am thinking about how the Bat-pod actually works in real life. The Transformer vehicles seem to have worked before just with added stuff on it. I could be wrong on this because I have not watched the makings of Transformers.
As for shutting down LA, they only really used one part of LA for the final battle which wouldn't cost as much as shutting down a whole city. The Air Force bases would be costly I would imagine but the desert in New Mexico doesn't sound like they are disrupting business there at all.
I think Optimus was rebuilt from a pretty much junked semi. But you're corect on the customization, the other vehicles were given to Bay by GM for free and customized (which is why they were all GM, he couldn't pass up the chance at free cars).
I'm not sure how many days they filmed in LA, and how that all works out with the city and such as far as a dollar amount. But its LA, I hear they have a department in city hall dedicated to working with filming productions, thats all they do. All scenes shot on Air Force bases were on the movie dime. Including time paid to real soldiers for acting, jet and helicopter fuel for any fly-bys, etc, so that no tax dollars were used for the filming of a movie.

