There are a number of reasons why crop frame lenses are less expensive, but as someone who has spend many years working in an engineering role in product manufacturing and mass production, I canít agree with your cost assessment. In general, crop frame lenses, versus pro lenses, tend to be slower (smaller maximum aperture) so the light does not have to be bent as acutely, and that is why the elements tend to be smaller and less of them. The construction tends to be cheaper as well. The actual material costs only make up a portion of the cost; labour, manufacturing overhead, R&D amortization and tooling amortization are going to make up a signification portion of all of these costs. Assuming that you sell 100,000 units of a crop frame lens and 10,000 units of a pro lens; if you sink the same amount of R&D / tooling costs into both, the crop frame unit cost will only have to carry 1/10 of those costs per unit. In addition, I would look at different types of tooling and manufacturing techniques for a production run of 100,000 units than 10,000 units, you start seeing some of the reasons for the lower costs.