Just my two cents, knowing a little about Scott Mann's business; remember that Sunset/3rd Rail/Golden Gate Depot is NOT a "high volume" provider of rolling stock. Thus, their "lower volume" lends itself to "hand made" precision brass steam locomotives and cabooses, for example. The ABS "plastic body" diesel models still have low volume brass under frames, and thus tend to be higher volume than the hand made steam models, but nowhere near the higher volumes that would be required to manufacture these modern freight cars being discussed. Therefor, the projected sell prices would be well above the $500 per set threshold, in my opinion.
I think your reasoning is correct, although I also think you might be 20-25% high, but then that's still well about $150. Than again, the specific trucks might cost $150 for a set of 4, 
I suspect that given a decent set of masters that I could cast components to make resin kits for close to $150 (minus the trucks), but I might be low on that guesstimate, and am also leaving out any profit to boot which would gravely upset my Ferengi accountant to no end.
