There were only a couple of offers on this recent thread.  Some of which were below peel at the time.  I only see these being peeled little by little unless they are transferred between people who know each other.  I know I wouldn't take the chance on a flight and a meetup with a random from the interwebz.

https://bitcointalk.org/index.php?topic=3323184.80

There's always someone richer, so 100BTC bars certainly are not nor will ever be completely unaffordable. It's a small market though. Something like 50% of Americans can't make an unexpected $500 payment. And USA has the highest national net wealth. I digress.

There is no "going rate" because any public auction is few and far between. I believe around 2015 Casascius (Mike) helped a friend sell a 1000BTC coin, at the time around $2 million or so. Was a public post on Collectibles. This is all pure recall so details may be off by quite a bit. Someone else would need to confirm.

Easiest is to simply create an auction here. You would be better off using your original account with established history/reputation. It would also help to have a PGP signed custody history from Casascius (proving he sold the bar to you directly).