It's not just "Doom is cheaper". It's "Doom is cheaper, Doom is an all-time classic, and Youngblood had a mixed reception".
I think the idea is that Youngblood being behind Doom is an indicative that the game is not doing well. The previous post gave some reasons to why that might not be so, but who knows. In other words, I believe this might have more to do with Youngblood's failure than anything else, and that's why the comparison is valid.
It's not like saying a game sold less than GTA V.