30%, as is standard. Disregarding the matter of whether or not it's wise to forgo a service with 65m+ users in an attempt to prop up your own platform (which has been addressed already), there's nothing preventing a publisher from selling Steam keys themselves -- which are provided free-of-charge -- and leaving the game off the store proper. So again, it's petulant politics.
They will still have to pay 30% to steam for a game they are publishing.
If EA publishes a game and sells it for/on steam they will have to pay 30% to valve. If they sell it on origin they get to keep those 30% for themselves.