I have'nt yet found any lenses in flea markets but I find lots of bargains on ebay.

The Pentacon 50mm f1.8 was a great buy and they ofetn go for peanuts on ebay....I paid just £6 for mine and that was including the postage!
My Taylor-Hobson 12" (304.8mm) f4 was another steal at just £36 inc postage, and I bought a Minolta Rokkor-PF 58mm f1.4 the other day for just £14 inc postage.
My Enna Tele-Ennalyt 400mm f4.5 was a bargain too at just £40 inc postage.
My Helios-40 85mm f1.5 cost me £56, my MTO 1100mm f10.5 cost me £60, and my Vivitar 35-200mm f3-f4.5 macro was £18.
I also have a 24mm f2.8, 28mm f2.8 macro and 35mm f2.8, and 200mm f3.3 which were all less than £10 each.
My most expensive lens to date was one of my Mamiya 645 medium format lenses, I cant remember which one, but it was about £130 inc postage.
However my sharpest Mamiya lens, the 80mm f2.8 cost only £59 and that was from a camera shop!
So all in all I have probably spent about £700 on all my lenses which is less than I'd have to pay for a single Sigma EX zoom lens!!!