House of the Dragon cost under $20 million per episode because HBO has become adept at making world-building shows

Variety

The $20 million cost is more than the $15 million per-episode cost of the final season of Game of Thrones, but House of the Dragon was bound to be more expensive with all the dragons flying around. Variety reports House of the Dragon was able to keep costs from spiraling out of control not only because of its experience with Game of Thrones, but with more recent world-building shows like Westworld and His Dark Materials. The $20 million per-episode cost looks low especially when compared to Amazon's Lord of the Rings, which costs $465 million for its eight-episode first season, or about $58 million per episode (including startup costs).