I expect that most of the people in charge of the Republican Party are emotionally committed to the idea that they're the "good guys" in an important fight. That doesn't mean that they don't have a price, just that it would tend to be very high. And since the Republicans is a voluntary association I'd expect that any rapid shift in policy big enough to be worth the investment would see the collapse and replacement of the Republican party.

Exerting the kind of influence that the Kochs do doesn't just require money, it also requires decades of hard and patient work. You've got to create institutions that attract people to work and publish on those issues where they happen to agree with you rather than where they disagree with you. You've got to make politicians feel that you're on their side and that listening to your spiel is just a matter of friendship, and you have to have a mountain of one sided evidence that lets them believe (and if you're their friend they'll be wanting to believe) that your proposition is Best For America.

But just buying a political party? I honestly think that if you're trying to accomplish a bunch of stuff quickly a coup would be cheaper and more realistic.