Jon M. :
I would guess that there was some cost difference, though. I can see it costing practically the same amount to design a fairly modern 4-4-0 and a 4-8-8-4, and to make the injection- molding and die-casting tooling, although with more wheels the tooling may cost somewhat more for the Big Boy. Labor costs for molding would probably be similar; the larger model, with more parts, though, would have a more elaborate sprue, probably costing more to trim. It would take more material, as well, than the smaller model. The larger model would need more machined parts - 16 driver tires rather than 4, for example - and this would add to cost. Assembly would take much longer, and it is one of the most expensive processes in making something elaborate like a model. Finally, the larger model would have more opportunities for quality control problems and breakage, adding to its cost still more.
So, while you can't price them by the pound like potatoes (or prototype steam locomotives), the big models would indeed cost more to make, but it certainly makes sense that a builder would find more customers for a $300 Big Boy that cost $150 to produce than a $100
0-6-0 that cost $80 -- it's the same reason the automakers push their big vehicles...
Cordially yours: Gerard P.