Originally posted by benr Why the big price differences?
Besides the clear difference between the autofocus and manual version lens prices, the fact is that for 9 years from 2001-2010 the Pentax 645 system was "dead" in that there were no digital variants of the camera, and was relevant only to those who still wanted to shoot film; needless to say the lens prices tanked.
Ever since the D and Z gained popularity, though, Pentax began re-releasing their lenses worldwide, which previously were only available in Japan. Of course prices and exchange rates changed quite a bit since ye olde days and so the "new" film-era lenses cost a lot more than the old stock, and on the B&H site you can see that some of the FA lenses can easily cost more than $1000 a piece, whereas they used to be half that or less.
So long story short - the price depends on where/when the lens was originally purchased/built and whether the seller is charging popularity tax. As such it's possible to find the FA 150/2.8 for $800 on eBay as it is for $1,400 at B&H.
Once Pentax rolls out the (rumored) 80-160mm f/4.5 SR, I fully expect it to cost a few grand, as modern lenses usually do.