Ooooooohhhh ok. So it's the mana cost of playing new terrain that stacks. But only the most recent terrain is actually in effect. Ok, I'm tracking now. So, if there are two terrain cards in one zone, only the top card is in effect. And if I wanted to replace that terrain with a new terrain, I would have to pay the cost of the first two terrain cards, PLUS the mana cost of my new terrain card. If choose to do this, then the terrain card I just played is the one in effect.
Thanks for the clarification!