Having the game on the platform means they save money, the have easy access to the marketplace, which itself provides quality assets at reasonable prices that in turn speeds up development time and reduces cost, as well as relying on AWS from UE4 that really helps smaller studios get a great foundation for the online part of there games.
It's very reasonable things will be cheaper. I don't think the cost will be massive but shaving a few dollars here and there is certainly reasonable with dlc and etc