I think 50 mana is reasonable. Not only because the feeling of being powerfull, but because IT IS POWERFULL. I mean, making that terraforming should cost a lot of mana.
Paying 50 mana gets sure you will think twice when using it, but the most important is that it is a reasonable quantity of mana for the effects of the spell: making land from the sea? Lowering a mountain? For less than 50 mana? Ok, you can reduce to 45, but less than that is nonsense.
Apart from that, is the strategical use of it...to tell you the truth, I did not think about it. But since there are no boats or sea-transports, to build a sea or mountain defense surrounding player empire gives to much advantage...the only way to avoid it (or not over use it) is by paying a lot of mana.