500 Mana sounds pretty darn cheap! 
Although the number of turns sounds about right, unless my mind is playing tricks on me but in a normal game of MOM you'd end up with at least a casting skill of around 200 (dpending on playstyle) and so casting the 5000 mana Spell Of Mastery would take just that, 25 turns. 
And it's good that it breaks all the alliances. If anything, all the other factions should ally regardless of alignment and march to your capital/sovereign.