Prior to the EX, Sigma used to have two versions of this lens - f2.8 and f4.5 APO's. About three years ago, before Pentax and Sony DSLRs got popular and used lenses were really cheap, I got a copy of the 4.5 APO version in MInolta/Sony mount, which I would have graded LNIB, on screechbay for $81 ($93 with shipping). Such a deal!
I've been looking for one in Pentax mount ever since, without success. Ahhh, for the good old days!
Enjoy your lens - I hope you get to make good use of it.