Actually a NSX with a slushbox can be had for about the same as a new accord. (like early 90's) To make copys, show the crappy auto's get blown up, beat up, peed on ect. Then for the Race parts, only on the entior shots, use the Stick version. Plus in they spend like 10k on custom body work and shit like that, it would not be that hard to pull it off. They even have r34 skylines in it, i think that they can afford some used NSXes.
went to mynsx.com and checked up on it. turns out it is the same car. guess the guy isnt full of it after all. but yea, i agree that they could easily afford to copy that car if they wanted to use it. maybe the producers told them that as a copout.