$75 for just levels. The game doesn't come with it. That's asking more than a AAA game.
Direct download link: https://romero.com/s/SIGIL_II_V1_0.zip
Clearly, many gamers don't consider the amount of work and resources that go into a game being the decisive factor in its price!
With that framing, Romero's pricing makes perfect sense. It doesn't much matter if he's designing levels for a game rather than developing a game, because the resources that go into the game's production are irrelevant to the cost.
What matters more is that he's selling to an audience that doesn't need to be convinced to buy his product. Unlike AAA games, who are targeting the marginal player—that is, the person who isn't necessarily going to buy the game anyway—Romero is selling to true believers (in him, in Doom, in their nostalgia for their childhoods, whatever). The cost is extra irrelevant to that group.
The price of a video game is not a function of how much it cost to make it. The price is set by maximizing unit price * expected sales where expected sales is a function of cost. The demand curves are cultural and that is how it works for any industry.
>despite the fact that the previous standard price dated from a time when game budgets were less than half of what they are today?
Conversely, gaming is a mass market that appeals to a larger amount of people than it did in the past. The masses may be less willing to spend as much as an enthusiast.
>What matters more is that he's selling to an audience that doesn't need to be convinced to buy his product.
He chose to make it a niche product. It would not have been that much extra work to make it into a full game and release it on Steam. He could have even outsourced that work. There are freely licensed doom clones that could be used so it would not need much development time.