I was out taking pictures earlier tonight with a mint condition £2 Super Travenar 28mm lens that I cleaned the crap off using Zippo lighter fuel and soapy water!
It's not a famous or sought after lens, which is why it was in a box under the table at the local camera fair.
I got a Tair 300mm ( mint ), Rikenon 135mm ( battered ) and a Helios 50 ( v. good ), complete with a Zenit camera ( door stop, but working ), for £45 at an antiques place.
There was probably 8 or 9 M42 lenses / cameras at the small boot sale I went to last weekend, and nothing over £10, just nothing that I fancied.
I dropped very lucky with the Tair, but the rest of the stuff is perfectly useable and makes photography a bit more interesting than the "spend lots of money on the newest, fasted, bestest lens" approach.