Probably the seller is a member here and is well aware of the value.
He's just not desperate to sell and trying to catch some less informed buyer with too much btc.
These are rare after all.
Rarity alone does not make the case for obscene prices, save for a very few examples over in the "real" coin collecting world.
The 2011 error coin is by no means rare, it is one of the largest mintage numbers for Cas coins, yet it sells consistently for a higher premium per coin than the 25
BTC, which is indeed a lower mintage. There's just not as much love for the 25
BTC than there is for the errors, or for most of the rest of the series for that matter.
The market for a 25
BTC Cas is roughly 26-28
BTC at the moment. Any user frequenting these forums is probably aware of that.
That user is just looking for a sucker who knows nothing about them and has money burning a hole in their pockets.