What the heck does "I'm buying gravity" mean? Well, if you're referring to the line in Defying Gravity, it refers to the line preceding it. Here it is in context:
"Too long I've been afraid of
Losing love I guess I've lost.
Well, if that's love
It comes at much too high a cost!
I'd sooner buy
Defying gravity"
She'd rather fly away than face her true emotions.
I don't think Wicked's lyrics are as horribly atrocious as some people make them out to be (and not sure how they "break" any "rules"). I don't think they're the greatest I've heard, but there is a good deal that is witty, clever and appropriate. I've certainly heard much worse.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian