The Canon 50/1.8 is probably your first best choice, but it's going to be at the top of your price range or beyond unless you get lucky. Canon and Serenar lenses are optically identical.
FWIW, I happily used a Jupiter-8 on my Canon P for a while before I got a Canon 50/1.5. Buying from a reliable seller (Fedka is one that has an excellent reputation) should ensure you get a good one. The discrepancy between the Leica standard and the Soviet (Contax) standard (described well by Dante Stella - worth reading if you haven't already) is very minor; on an f/2 lens like the J-8 you will likely never even notice it, and if you do it will only be wide open and close up. Within your desired price range, the Jupiter-8 is what I'd go for (and I did).