If you have the SMC M 50mm F1.7 you really got a great lens.
I did! I was at the market, and one vendor was selling (amongst assorted wares) a camera with a couple of lenses "for parts or repair." She wanted £12 for the lot. There was a Pentax body, attached to a rather cool Pentax zoom lens. This had obviously been dropped; there was real damage to teh filter ring, a few scratches on the objective, the zoom part of the lens freely moved an additional centimetre in and out (which didn't adjust zoom, but did wreck the focus whenever it moved) and the body itself wouldn't properly cock the shutter when it was wound on, and the shutter speed dial was stuck fast. Sat next to these two write-offs was a really badly fingerprinted and slightly scuffed 50mm prime.
"How much for just this bit?" I asked.
"Oh, I want rid of this all together. I'm not going to split it."
"I'll give you £5 for it..."
"Ten for the lot."
"OK, I suppose."
The prime cleaned up nicely. No fungus, one nearly invisible ding on the glass, and mechanically in excellent condition - obviously not the bit that was dropped! It only had the back cap, and I've bought a front lens cap for it for 99 pence. It is exactly the lens you linked to, and I'm really looking forward to my adapter arriving in the post. It will be my longest lens, whilst ironically being the shortest, physically. The kit zoom is longer when extended than this lens will be on its adapter.
So, for £10 I got a nice lens case (to fit the zoom), a nice "For Pentax" neck strap, a lovely prime lens, and a body and zoom which have been given to a friend of mine who likes to turn broken technology into steampunk jewellery.