Honestly, ive spent way too much time tracking price data on these. If you're on a budget but want that medium format look, look for the Fujifilm GF 50mm f/3.5 R LM WR on sites like MPB or Fred Miranda. I've seen them go for around $600-700 used. It's the cheapest way into the system and the resolving power is high enough to outresolve the sensor on your 50S. Over the years I've tried many of these and the Fujifilm GF 32-64mm f/4 R LM WR is the real bargain if you find it used. It's usually around $1100-1200 now. That linear motor makes a massive difference for the contrast-detect AF on the 50S... helps a lot with hunting. Just watch out for the weight since its a bit of a brick compared to the primes. If you want more technical breakdowns on the MTF charts or corner sharpness for specific focal lengths just ask, I've got plenty of notes from my own testing.

ngl it has been kinda rough lately finding good deals that arent beat to hell. i tried getting some glass through local classifieds and even some smaller regional forums but honestly had issues with the condition not matching the descriptions twice in a row. it was super disappointing because i really wanted to save some cash for my kit but ended up just wasting time dealing with returns and shipping... i've found that some sellers really gloss over the mechanical wear on the mount or the rings. are you mostly worried about the glass quality or do you care more about the cosmetic condition too? also whats your actual budget range for this? knowing if you need a walkaround lens or something for portraits would help narrow down where to look.

Just do some searches at Amazon, eBay, Keh and B&H Used store.

I used to get these lenses at Amazon warehouse deals.