Conclusions I've seen from previous threads:
Leaving aside design, for a moment... Materials, methods:
Materials costs are not just the base materials, but also the fittings, finish etc.
Methods; not just is it cast, machined, forged, but also how much finishing (and to what level of polish) takes place, straightening, QA(!).
Then there's design. You'd hope that a higher end riser has sunk a lot of cost into this, but it's largely upfront. So when you're buying the product, you can see that either as recouping sunk design costs, or financing the next iteration.
A low priced riser... you can cut materials costs a fair bit, but only so much (just not that much headroom), a lower grade of metal, cheaper methods... but the biggest cost reductions are probably in process - taking a knock-off design, cutting out finishing steps, skimping on QA, cutting out after-care.