Tanks @Pontac and Daisi. I did a little research a couple of years ago and Avalon and Viking were my choices. Other lines seemed more 'plush' and more expensive, and the differences seemed mostly related to the ship.
Avalon seemed to stand out over Viking in my analysis at the time, better cabins, smaller passenger size etc. And prices were comparable. So I was surprised when I started looking again for 2025 that there is a big price difference.
It did not seem to be just one cruise itinerary / date. There were offers of free or low-cost air etc. but I tried to stick with just the basics.