Open the directory,where you installed it. For example: 'D:\BW2\Data\Balance', As you seen, enter to the Data\balance menu. Find the text file: GameBalanceMiracle. You can see the miracles listed with numbers in columns. We just need the first one:

e.g: METEOR 7500

You can edit it as you wish:

e.g: METEOR 10

So the meteor miracle will only cost you 10 mana points :) Enjoy!