Originally posted by link81 Pretty certain that the only WR lens's yo'll find with a pentax mount will be pentax lenses. I don't believe that any 3rd party WR lens's exist.
The cheapest you're going to be able to find will be the 18-55WR, they go up in price from there, with the 18-135 and 16-85 being the next step up, both in quality and in price.
here is the truth, for WR reject Sigma, Tamron and Tokina in K mount. They don't exist as far as I know. the reviews here don't lie.
Save your cash for the best lens you can get since your K-5 is very capable. Look in the forums here in the for sale thread, it will give you ideas on actual cost for used (or experienced,) lenses will cost. Then cross reference the reviews and you can make an informed decision and spend your money wisely.
The DA 18-55 WR is pretty good as are pretty much all of the newer generations of lenses with WR. I have seen the 18-55 for under 60.00 USD on KEH. Good luck!