I don't know, but that's never stopped me from speculating before:
Focusing precision. The focusing cam, focus distance scale, and actual helicoid(s) need to be precisely aligned in a rangefinder lens. SLR lenses have quite a bit of slop -- even among my Leica SLR lenses -- but that doesn't matter because focusing is achieved via ground glass, not mechanical linkage.
Size. Elements in smaller lenses do more optical "heavy lifting" than in larger lenses. Consider two 35mm f/1.4 lenses, say the Zeiss SLR lens for Contax/Yashica and the Voigtlander rangefinder lens for M mount. Both nominally take the same amount of light and bend it to the same degree. However, the smaller lens must achieve this with fewer and smaller elements, meaning that each element is required to do more work. This leads to both material selection and assembly issues. To achieve the same quality with less glass means using less common, more expensive glass types and curvatures. Similarly, the misalignment of a single element in a 3-element lens is more likely to be visible than in a 30-element lens, because each element in a "simpler" design is asked to do more work. So, making a smaller lens is more difficult, even if everything else is the same.
Today's manufacturing technology favors autofocus lenses. In terms of structural materials, brass and plastic both allow for extremely precise and reliable lenses, but plastic molds only become cost viable with high volume production runs -- but yield very low per-unit costs with very high volumes. In terms of mechanicals, brass helicoids require expensive manufacturing and assembly to achieve reliable and precise focusing, especially with multiple simultaneous movements (e.g., zoom lenses or floating elements), while autofocus lenses with linear motors can achieve sub-micrometer precision using automatic calibration and nearly haphazard assembly. Moreover, while rangefinder lenses could be made using SLR technology in the 1970s, building them today enjoys fewer and fewer economies of scale, further increasing the cost disparity.
Finally, I'll offer the "there must be a difference -- economics says so" argument. The concept of supply and demand says that prices will be as high as buyers are willing to pay and also as low as sellers are able to sustain. If prices are unnaturally high because there's lots of demand but a vacuum of supply, then new suppliers will enter the market. If suppliers aren't entering the market, it is because potential entrants have considered doing so and found that it would cost too much to offer a product for sale. So, while we've seen a few new companies enter the SLR lens market (e.g., Samyang, Zhongyi, Yongnuo), we've also seen Zeiss leave the rangefinder market. All this tells me that there "must" be something that makes rangefinder lens production inherently more expensive. ;-)
But I don't know. All I'm really sure about is that using a rangefinder is a good fit for me, and I'm willing to pay a premium to continue using that method of seeing and focusing. I don't think it is coincidental that the high precision requirements of the rangefinder focusing mechanism seems to be correlated with high quality optics, but I'll continue enjoying that correlation even if it is purely random.
Cheers,
Jon
