Khloe Kardashian and Mario Lopez To Host X Factor

We broke the story ... Khloe and Mario had been thiiiiiiis close to a deal for months ... but now we've confirmed they've both signed on the dotted line ... it's a done deal.

We're told the duo's salaries will add millions to the budget -- on top of an already enormous talent budget ... with Britney Spears in the lead at $15 mil.