Vivitar 135/2.8 Available in both M42 and K-mount. Should be available for under $50 USD.
My review of and some images from an early M42 version:
http://www.pentaxforums.com/forums/p...rst-three.html
http://www.pentaxforums.com/forums/p...135-2-8-a.html
Steve
P.S. The lens looks huge in my product shot, but is really quite compact.