I am sure that I am in the minority, but I have to say that I hate this.
Magical energy necessary to change 100kg of dirt into mud: 8
Magical energy necessary to raise thousands upon thousands of tons from the bottom of the ocean: 5
Terraforming spells should feel epic, and at 50 mana they barely did. At 5 mana, they are a gimmick. Oooo, see how we have unreachable areas with goodies. Oooo, check out the spell we have so you can reach them.
And don't bring up how city enchantments are cheap. They are fueled by the the magic of the land, over a huge area, over time.
If you need it that badly for the campaign, tie it to bloody item, an artifact tied to the spirit of WhatHaveYou, and have the protagonist find it.
Not so sure you're in the minority here, old chap.
I couldn't believe my own eyes when I first saw this change in mana cost, my jaws dropped to the floor. The developers have done so many reasonable fixes and changes to balance, I didn't expect them to do something like this. In fact it's really the only thing done to this game the past year that makes me sad.
Besides balance too, it just don't feel right due to the epicness of this spell, as Tuidjy mentions here. It's far away from the scope and the pasting of the rest of the game, and makes this spell cheap as a hokus pocus trick (but powerfull indeed).
Hopefully the AI won't use it, because I won't myself. I just don't see the fun in being able to do this stuff practically for free. I'd rather see it cost more then 50 mana too, but guess 50 mana might be an OK comprimize (and Earth level 3 at least).
I believe the Frog to be on deep water with this one.....