Best value is surely Tamron SP Di 90mm F/2.8 Macro. A great lens, tack sharp, auto-focus with focus limiter for when you're not shooting macro. 100% recommended. Can be found brand new for quite a bit under $400 if you look a little (e.g. Prodigital2000 on Ebay).
For less money, I'd recommend Pentax-M 50mm F/4 plus extension tubes. Manual focus and you'll need to meter with the green button, but as long as you get tubes with a aperture lever, at least you can focus wide-open. Around $100 for the lens, maybe $50 for tubes with an aperture lever. With this combo you'll be able to exceed 1:1 macro too, but you'll lose infinity focus...
[EDIT:] See some of my macro shots
here (best ones near the end)...
Last edited by Doundounba; 09-30-2013 at 08:08 PM.
Reason: add link to macro galery