Some good old Pentax manual lenses cost more due to rarity, or at least perceived by the market. Also, there are rich Asian collectors who are willing to pay more. But the truth is, old lenses just built much better than any Pentax lenses you can buy today with far less optical misalignment and absolutely no BF/FF. Boy do I hate optical misalignment? They are also great to use. The feel of turning those smooth focus rings is so stimulating.
And if you want doing video, the smooth action of MF lenses is much preferred anytime.