I believe when you sailed in February 2023, the submersible ride was free or included in the fare. I can imagine the demand must have outstripped the supply of seats, with lots of disappointed guests. Viking changed the rule in June and for our sailing, they charge $499 a ride, most likely to mitigate the issue and complaint.
I've watched a couple of You Tube videos of the sub ride. I'll be honest, what you can see on the Antarctica sea floor is really underwhelming to me. Maybe my expectation is skewed by the colourful coral and fishes that you frequently see on those brilliant National Geographic TV programs.
Did you get a chance to ride the sub yourself? Would you have done that if you had known that's what you'll get, and had to pay $1K for 2 for the "been there, done that" bragging right. I know $1K is nothing compared to the cruise fare in the grand scheme of things. I suppose this is all personal too.