Wanting a "cheap" 50 (I bought the 1.2/50 new for just over 400 euros last year) for my FM2n, FE2, FA, I got myself a 1.8 Ai-S for 50 euros. I was really looking for a 2.0, but couldn't find it.
That 1.8 is reasonbly small and optically a great lens. I've used it a lot for outdoors portraits with Tri-X, close up and personal at f2.0 or 2.8, and it's IQ is amazing for that sort of work. Really sharp in the centre, great out-of-focus rendering. You can't go wrong for 50 euros!