unless it's a rust-bucket, the price isn't too far off - and yes, you're paying that extra several thou of a premium for a real Cheyenne Super swb. If YOU personally don't care if it's a real Cheyenne Super or swb - then it's probably not worth the extra several thousand you're paying for what the SPID says that 95% of buyers don't care about (I'm in the 5% some days, the 95% other days) But without pictures, without an understanding of why it isn't running, everyone - including me, is guessing at this. Non-running could mean anything from the battery is dead to a rod is hanging out of the block. I'll say this much - the Custom Two Tone paint does add value, 557 is not common it looks awesome.
IF the engine is the original (or could be) and is repairable,
and
IF the body doesn't have much more than surface rust or the usual rocker rot,
then price is probably pretty fair for what the truck will be when repaired.
It just comes down to what you want and the condition. Personally, a 557 swb super is a bit of a grail truck in my opinion, even if it's not a big block. But I still can't tell you if I'd want to spend that much until I saw at least a few pics. Whatever happens - nice find and good luck!
Let me tell you a story... I passed on a 72 Super swb big block mainly because the seller refused to let me crank the engine when I was there in person. BUT, the price was fair even if it needed a total rebuild. I passed the info on to a fellow member, he bought it, and it started and ran just fine (seller truly was just worried about damaging it because it hadn't been cranked in years - I had a great relationship with the seller, he was a great guy) - he had his fun with it and sold it and, I'd imagine, he made a decent penny on it. I've regretted not buying that truck every since. Solid ochre too...
