Fair price is whatever the buyer is willing to pay and whatever the seller of the product is willing to accept.
I don't agree.
Humans (and many other animals) have an inherent sense for fairness and fair deals. If you have the feeling that a company demands unfair prices for a product, this will effect your longtime opinion for the company and you will probably be more cautious in the future when making a strategic decision like buying/preordering a game. It is easier to preorder when you have confidence that the company will offer fair deals in the future.
On one side the developers make all those efforts to really interest community in Civ 6, DLCs, Expansion with lots of youtube videos, etc. ... and on the other side they tell people : you bought Civ 6 standard on release so you will have to pay full price for all DLCs or you can buy the Deluxe Edition (in addition to your normal one) for 2 $ more.
Civ 6 Deluxe = Base Game + 20 Civs + 7 Civs (DLC) = 40 $
6 DLCs = 7 Civs (DLC) = 38 $
-> Math : Base Game + 20 Civs = 2 $
If the developers are really the nice guys from the community videos, then they should offer a fair pricing for their fans, independant from the fact if they bought the standard edition or the deluxe. For me Civ 6 is the game where the devs denied the option to buy a Season Pass to players who preordered/bought the normal game. And they did not correct this mistake in one and a half years.
I'm sure that there is a simple algorithm to generate psychological "fair" prices ...
Regarding the DLCs :
Actually the market is not willing to pay that much. Look at steam reviews :
Civ 6 : 69%
R&F : 62%
Australia : 39%
Khmer and Indonesia : 48%
Nubia : 37%
Persia & Macedonia : 34%
Poland : 30%
Vikings scenario : 9%
Based on those ratings they should have included all the DLCs for free in R&F and closed the DLC-chapter.