Airports are always expensive places for coffee and other refreshments, though some are more expensive than others.
This morning I'm en route to Tivat, via Paris, so I'm having coffee in terminal 2B of Paris CDG.
There is only one cafe, so they have a monopoly on all passengers who are "airside". This monopoly is reflected in the prices:
small bottle of water - between €2.90 and €3.40
Cappuccino €4.50! (and that's not a tall one)
If the cafe had some competition I suspect its prices would be a LOT lower, though the cost of doing business in an airport (rent etc) would probably mean that the prices were still higher than what you'd pay "downtown".
Competition in any market is a good thing. It stimulates and motivates companies to provide better services and products to retain and grow their market share. Consumers benefit.
Pity the same rules don't apply in CDG