Map-trading was fine in the older games, except for one thing. They were too cheap.
To trade an empire-map for an empire-map, should be based on size. If they're bigger, you have to put in some gold to know the rest.
IF you want to know what they have found, then you should have to put a penny for every tile they've seen and I don't mind this as to be random. So all in all, this could be an expensive way of discovering the world, but it should be in there and properly legal.
Even maps for techs was fine in the old games, but maybe now maps +gold for resources would kill the game? It's still the 30-turn deal and who knows who profit on it? You or the AI?