In Civ 6, the cost of religious units seems to go up the more you build. For example my first inquisitor was 100 faith, and the next one will be 105 faith. The same happens for missionaries and apostles, but I haven't kept close enough track to see what the algorithm is.
It also seems like the cost might go up over time or based on other factors. The base cost of an inquisitor is 50, but as I said my cost was 100.