That depends on how you want to make them and if they are completely new including wheels and axles. Recreating exact replicas, which is what these are except for the boxes, is the most expensive. Considerable savings can be had by using different materials and fabrication methods. All steel construction would probably save money. If enough were being built, box castings could save money. I believe D&S fabricates all their boxes and they seem to have it down to a science. I don't know what they cost to build. I seem to recall seeing a one piece side frame and pedestal casting in Chama one time. I don't know if that project was ever completed. John Bush might want to weigh in on that.
That's the long story. Short story is they won't be cheap.