@Kongdej
The mountains are 4 tiles across so you have to do a line across them - i.e. you have to cast it 4 times just to get across one mountain range - 200 mana
And I will gladly pay that for the powerfull ability to cross a mountain, that said its another reason why "Master Scouts" is slightly broken imo 
I played enough games, and used raise/lower land to be aware of the mana cost, and I think its in a fine place atm. Power comes with a cost.
That said I miss being able to create mountains, and hopefully someday create ocean back up.
@ Cohol: Are you considering the power of permanent boosting a champ?, destiny's insight IS useless though, 25 xp for 250 mana is... worthless 
Destiny's gift need to cost some heavy amount of mana so you wont just spam it on all your champions, or be limited in some other way.
I am not trying to be a devils advocate here and just counter argue over what you guys are saying, I think there are plenty of spells with a waaaay too high mana cost, I am surprised the mana cost on flame dart have risen so much over the last beta.
I agree that bless city, incinerate city, and some other spells that I need to be looking at before mentioning is waaay too costy compared to what they do.
Just raise/lower land should be "somewhat" expensive, so I can't just spam it all around, and the same with destiny spells.
Sincerely
~ Kongdej