The most likely suspects have already been listed, but just wanted to throw in the Summicron 40/2 or the Minolta Rokkor 40/2 in your price range. I recently picked up the Cron for $450, and KEH has a Rokkor listed right now for $415, a reputable lens.
I list the 40mm b/c you have an M8, so with the crop factor, I think this would work out to be a little over 50mm.
But, this is only your first 50mm, and.........you.....will.....want......more.....50s................
good luck