Is there any macro lens for around 70$ pentax mount with focal length around 50mm?

Answer 1

Sigma makes a 50mm f/2.8 macro AF that you *might* be able to find used on eBay or used somewhere. New, it's about $250, but used, you might be able to find it for less. Not too many out there though, so finding it used can be hard.

Out of curiosity, why a 50mm macro? The working distance is very short, so that it's not good for bugs or things that can escape. It is good for things like flowers or other static, non-moving objects.

A less expensive alternative would be close-up lenses, previously mentioned to you. You could also look at Pentax extension tubes, which will give you increased magnifcation for macro work:

example:
http://cgi.ebay.com/Extension-Tube-Set-Marco-Ring-for-Pentax-K100D-K10D-A75_W0QQitemZ190170797665QQihZ009QQcategoryZ50489QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Answer 2

Pentax offered a 50mm macro which you can find on ebay and in your price range. Just be sure it includes the 1:1 tube so you can take life-size images. It will be manual focus.