just get an adwords account and use the keyword tool. It will literally tell you how much the top spot costs.
But top paying niche doesn't matter if you have no knowledge/passion for that subject. I'm sick of that top paying keywords search, be a leader in your niche, whatever it is and you'll make money.
No matter what my previous answer was, use this: https://adwords.google.com/select/KeywordToolExternal No need for an adwords account.