Originally posted by Aristophanes
The only way to look at the prices is compare and contrast:...
This is a fair point, and some of your comparisons make sense, but perhaps the best approach is compare lenses with the same optical design. Some of the best Pentax zoom lenses are available for Nikon and Canon via the Tokina "Pro" series, which developed these lenses jointly with Pentax. Let's say you want a wide-angle zoom with the performance of the DA12-24 or the DA*16-50. Let's compare the Pentax to the Nikon-mount options (all B&H):
Pentax SMCP-DA 12-24mm f/4 ED AL (IF)
$897
Tokina 12-24mm f/4 AT-X 124 AF Pro DX II Lens
$449
Pentax SMCP-DA* 16-50mm f/2.8 ED AL (IF) SDM
$1297 
Tokina AT-X 16-50mm f/2.8 Pro FX
$749
The Tokina lenses are pro-grade lenses with metal bodies. There are very well made. But, some may say, "the Pentax versions have better coatings!!" I would say that there's not much evidence of a significant difference, as Tokina's coatings are very good. But, some may add, "The Pentax 16-50 has SDM!!!" To which I would reply "ADVANTAGE TOKINA!!"
I have not had great luck with Pentax quality control. I see no point in paying a $450-$550 premium to get a lens with the same optical formula, and that is no better made (and may be worse). The price differences are far too great.
How many people would buy the Pentax lenses at these prices if they could buy the very-similar Tokina lenses for a fraction of the price? If you wanted both of these lenses you could buy the Nikon versions and use the savings to get a "free" body!
Dan