Well, there's more to it than that. Supply & demand enters into the equation, of course, but it isn't the only factor. Sure, the difference in basic material cost for making a wider rim is small, but it is there. But the biggest influence in Niche product pricing is typically the actual volume produced.
What a lot of people don't account for is that setting up production for a product isn't cheap, regardless of whether it's wheels, widgets or whatever. The set up costs are more or less the same if you're making 5 or 5 million. Consequently, tooling and set up for small volume represents a huge part of the per-piece cost, but only a small fraction of it when you can amortize it out over thousands of units. Also, in very high volumes, more expensive but much more efficient production methods can be justified, which generally reduces the per-part cost even more, but niche market production cannot take advantage of that.
So, niche-market products are inherently more expensive to produce, sometimes significantly so. Then, any profit and supply/demand issues are factored in on top of that. The bottom line is that if you want to play with real unique things, you have to be willing to absorb the cost of doing so. Any other expectation is unrealistic from a manufacturing point of view.
