Hmmmm, I think it should be cheap when new; otherwise the guy who found the Leica M3 and Summicron for five (pounds, dollars, euros, roubles etc) will win the prize.
So far I reckon the Smena is the cheapest or else the Cosmic Symbol. They were pennies under eight pounds the last time I bought one new and two for a pound not so long ago in a junk shop (but he must have been reading RFF before pricing them).
Regards, David