The thing is, I don't think the mana cost is the problem. It's how quickly the spell is available.
I was in a game last night where my starting location was on the western edge of the map with a 25 tile long mountainous region (12-13 tiles north or south either way) on the east. That's a long way to walk around with your first pioneer or hero to scout out. If I'd had lower terrain, I could have cut myself a path pretty quickly to a nice fertile land once I'd made that first pioneer.
But sadly I had no Earth magic, so Ctrl + N it was. The point being is if I had early access to lower terrain, I'd have had a nice game there - everything was sweet on the other side.
So it's not early access that's the problem.
The problem, as I see it, is that mountains can't be crossed because there are no flying units and you don't have boats (or swimming units) yet the game insists on using lakes and mountains as internal terrain. Either you accept that you will continue to have this geography, and make lower terrain and raise terrain as accessible as it it now or you re-write the mapping code to not use mountains and internal bodies of water.