Not sure about the Rent but if you look at some of the other stadiums that have been getting upgrades the past few years $85M for 5k doesn't seem that far off from the standard since they usually also include luxury seats and other upgrades.
UNC paid $70M for 3k seats just a few years ago while Texas A&M's in the middle of a $450M project for 20k seats
On the flip side, ECU only paid $20M for 7k seats but I think that was only possible because they replaced one of the scoreboards with additional seats so they didn't need to build a whole new structure to support that.
Raw seat additions is the wrong lens to view it through.
This expansion was about PREMIUM seating that will kick a few million more in annual revenue back to them and grow their athletic budget.