What is a reasonable price for a Pentax 100/2.8 SMC-A Macro?

I'm wanting to do macro photography with my K1000 and the 100/2.8 SMC-A Macro lens seems like the ticket here. Pricing information seems hard to find, or I'm just not looking in the right places. What would you folks say is a reasonable price for one of these?

On another note, do you think I'd be better off with my 50/2.0 and the Auto Bellows?

Thanks for the help!

I've tried Standard 50mm f1.8 & f2 Pentax lenses on bellow (non auto) and while the results are OK, I found that my enlarger lenses worked far better as macro lenses. You need to reverse a standard lens for Macro work.

Of course the best quality is obtained using the Pentax Macro lenses, in the past I've used the screw thread 50mm f4 SMC Takumar Macro and the 100mm f4 Macro both are far superior to using the standard lenses.

There is also an excellent Tamron Adaptall SP1 90mm Macro lens, and it is a superb lens if you can find one, I kew two professional wild life photographers who used them.

Ian