I also find it difficult to believe it’s more expensive than the average TV drama, considering Fincher already has a development deal with the network.

I'm not saying you're wrong, but Fincher is very particular about his shots, and as such there is a metric shit-tonne of CGI in both seasons of mindhunter. There's a few videos on youtube that demonstrate it. Again, I don't think that would inherently make it way more expensive than the typical, but his vision comes with a price tag.

It’s like Netflix has never heard of a “loss leader”. People don’t pay exorbitant subscription fees to watch the latest Project Runway knock off show.

Netflix is notorious for spending a metric fuckton of money to the point that a great deal of their shows are "loss leaders" (and usually followed by prompt cancellation). The "Project Runway" comparison is irrelevant - although Netflix has plenty of trash reality shows, by no means that's all they do.

I also find it difficult to believe it’s more expensive than the average TV drama, considering Fincher already has a development deal with the network.

I don't have a good sense whether Mindhunters would be expensive or not, but my anecdotal experience says it's not a mainstream show. It's just not the hits like Stranger Things, Wednesday or perhaps the One Piece that Netflix needs to survive. 1899 is another victim of the Netflix gambles - an extremely stylish and intricate show that died a premature death, even though it launched to great reception.