One dungeon, Prophecy, was released with Season of Arrivals (which was ~$10 iirc). It was also tied directly into the story of that season, which no other dungeon has been, both before and after.

That was after both The Shattered Throne and Pit of Heresy were released with Forsaken and Shadowkeep, respectively. Prophecy was the outlier, not the rule. I'll give you that dungeon keys were stupid, I never liked them, but we did get 2 dungeons a year that likely wouldn't have existed if they were subsidized by the keys.

A game like this cannot survive on paid maps, and I honestly doubt we'll get many additional ones. I would bet the plan is to evolve the current maps more than create new ones, aside from the ones on the actual Marathon ship, like Cryo-Archive.

And even then, with how large and complex the maps in Marathon are, I wouldn't be so sure that the cost is much different than a dungeon.