Well, if the amp is the same, and the driver is of similar quality (it might, out of necessity of it's smaller size, be of even higher quality than the 12" in the 500) the only cost difference in building the models is the smaller cabinet of the 400...which in the big scheme of things is likely rather insignificant to the total cost of production.
I'm not playing fanboy; it's pretty well known that the 500 is not the cheapest alternative out there. I'm just saying that I wouldn't be surprised if the cost to construct one was $100 less than the 500.....
