We can't be sure since we have no access to the controlling code that causes the warp-around effect back to a small or negative cost in beakers, but anything equal to or greater than a cost in beakers of 256,000 (for Vanilla) is probably going to exceed the limit. I believe with Rise and Fall the number is even smaller. Remember the cost that is used in-game is also affected by game-speed. Marathon game speeed for example already multiplies everything by a factor of 3. I don't remember offhand whether additional modifiers are already implemented by Civ6 for player difficulty setting.
The issue has to do with Firaxis (or any software company for that matter) has to allocate ram-size for every element in the game as part of the game's controlling code, and in order to minimize the amount of RAM needed (and therefore make the game run faster) they have to make assumptions about how large a number is a reasonable limit, and apply (allocate) the amount of RAM (and thereby the available valid numerical values) that they feel will give reasonable limits while at the same time not using more Ram than really necessary. After that numerical limit is exceeded by effects coming from mods, strange things can begin to happen, like tech costs reverting to a negative or "1 beaker" cost number. The quintessential example of this was in Civ1 where Gandi's liklihood to use nukes was programmed at the smallest value available, but Sid and his team forgot that when a player went for the Democracy Government, the initial setting was reduced by two, which resulted in Gandhi getting a negative value and this caused the original civ game to treat it as the highest possible value -- hence the Nuke Happy Ghandi meme/joke in all the succeeding versions of Civ.
