Hi Judith,
Probably not much help, but I noticed that the 180 Macro was disappearing from the big dealers over a month ago. I figured that Sigma might be discontinuing the model as it was (probably will convert it to HSM/OS, or just drop it in favor of the 150), so I started looking seriously for one, and was lucky enough to find an open-box one at Adorama. Although this was not a primary LBA target of mine, I immediately bought it, and am now glad I did. The lens was perfect, except for a small broken piece in the rear lens cap. I paid well under $700 USD.
Considering the way things usually go with premium K mount lenses, demand usually goes up when the word gets out that a lens is discontinued, so prices go up for used examples, as they have on the PF Marketplace -- the last examples I've seen there were being offered at close to $1K. I think that used lens dealers are the last to follow the trend, so I'd look at the used sections at B&H, Adorama, and of course, KEH. They would tend to mark down the used asking price from their last new selling price as opposed to pricing them according to the private market -- but they do catch up eventually. Lenses like this go very quickly, so you have to check often, and late at night and very early morning are times of the day that I have found most of the hard-to-find lenses. Have the funds readily available, and if you find one that seems to be acceptable, don't hesitate -- all three of these dealers have very good return policies, and the only thing you're really risking is shipping both ways -- a small price compared to the the cost (and value) of the lens and the added risk of buying from an unknown private seller.
Another possibility is to search the websites of really obscure online dealers. I've found quite a few deals from tiny dealers nobody ever heard of. A good example a few years ago is my first F 1.7x AFA, which I got for $75 from a small dealer in MN when the going market price was $175-200. Most of these dealers probably sell more C and N gear in a week than K mount stuff in a year, so the Pentax gear for them is an afterthought, and they don't bother to keep up with the market. You might be able to take advantage of this.
IMO, it's a lens that's worth doing some work to find if you have a use for it. I'm more than pleased with mine. Scouring the used (or even the overseas) dealer market for one will probably be worth it since a new HSM model will undoubtedly be more expensive (if it appears), and Sigma hasn't deemed the K mount worthy of their 150 Macro (I'm just being snarky -- the market for such a specialty lens in K mount is probably not large enough to justify Sigma making them, and with the world economy what it is, business is business)
Scott
