Nice coin! I haven't seen one of those in a while here.
1 BTC coins seem to go for around 1.05-1.15 BTC with ~0.02-0.03 BTC forks included
Considering the 5 BTC is more rare, 5.5 BTC sounds like a reasonable price maybe?
Unfortunately with current BTC prices this is way out of my reach

I have a lot of Cas, but not a 5 BTC. They don’t come up frequently. In today’s market I think a more accurate valuation would be 5.2, IMO, especially in that condition, but others who maybe have a pulse on recent 5 BTC Cas sales please chime in.
I picked up an MS68 5 BTC a few months back (pre ATH) for 5.9, so definitely agree that 5.5 BTC might be pushing it for an ungraded one in the current climate. 5.2-5.3 BTC seems reasonable to me