The wheels alone are more than 1/2 of the price.
You have to keep in mind that one wheel has more parts than the rest of the carriage. As an example, a 12 spoke wheel has 12 spokes, 6 felloes, the nave (hub which might be multiple pieces if large enough), four nave bands, dowels, the tire and 6 tire screws and nuts and washers. And if you are really making a repro wheel, there will be some metal innards for the nave, too.
True wheels are complicated.