Originally posted by kbrabble If you dont mind buying 3rd party lenses, you can occasionaly find the Tamron Adaptall 2 300mm f/2.8 on ebay, or at other used camera sites, for $300-400 and the pentax adapter for around $25.. I do not own this lens personally, but a friend uses it with excellent results, and there is not much, if anything, else available that is as fast in your price range..
The Tamron adaptall2 300)2,8 is also a super match for Pentax 1,7x AF TC.
The extra 1,7x is very handy when (not if) you need extra reach.
It will give you a 500/4,5 AF lens. With a bit of patience you will find the TC at a reasonable price at ebay.
You will find some exsamples of this combo at
http://fototim.photosight.org
I still have a lot to learn, but this will give you a rough idea of what the combo is capable of.
Fototim