Actually it seems the market for used Pentax glass is quite overpriced right now, because there's a free-for-all among the digital SLR buyers looking for lenses. Also, Pentax is discontinuing many of the -FA lenses but doesn't have their replacements in production yet. I would agree with the comment about screw mount lenses; the frenzy hasn't affected them so much, or you could look for Tamron/Sigma/etc.
My personal recommendation for a second lens would be a 35mm or 28mm prime, or possibly a short-throw zoom for the convenience (the Pentax-F 35-70 is a good example). Like csandlund said, the 135 is a good telephoto to start with, but avoid the common k-mount Takumars (the ones that have "Takumar" printed on the outside of the lens barrel) as these are a cheap 4-element design.