easy way -> (not available)
hard way -> make millions of copys of your spell for every mana it may need and create triggers to switch to the proper version
my suggestion: forget it, its not worth the work.
Like Accname said , there is no easy way. I played so many maps , so far what you suggested is still something impossible. Actually you make your spell's mana cost to 0. Then trigger it , but then it will cause alot of problem. For example , when your hero's mana is 0 , he/she can freely cast that spell without any mana.