A price difference cannot be "more expensive", just bigger.
The reason can be the lower market share of Pentax and therefore lower revenue from Pentax users. Other possible reasons: less developed dealer networks ..
But that's all only valid if you compare the same lens. Don't compare just similar focal length from different makers.
Because the cost of developing the mount is spread among fewer users. The same lens (eg Tamron 17-50) is distributed by the same by the same distributor regardless of mount so it won't have an impact on the price.