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.