How much would it cost to take over 51% of bitcoin mining?

How much would it cost to take over 51% of bitcoin mining?
Imagine a scenario where a very wealthy individual, corporation, or government wanted to take over bitcoin mining. How much would that cost to buy enough computing power to do so?
I don't keep up on all the mining costs, but I'm curious if we're still at the level of just millions of dollars, or billions of dollars to do this.
In other words, the amount of hardware, and cost dedicated to mining is a good approximation of who/what btc is subject too, and how vulnerable it is to "de-centralization".
We're currently fighting over how/if to scale bitcoin. But could the argument be over if Big Interested Parties suddenly became interested, sweeping away everyone currently involved in mining?

51% would be 70'000 Antminers S9, at $2400, gives 168mill for hashing hardware.
Before you start "crowdfunding" for a 51% attack, you should listen this 2 min video https://www.youtube.com/watch?feature=player_detailpage&v=yWTQgmCuiCw

Right now the active miners are executing about 1'600'000 trillion hashes per second (TH/s).
You can look for prices and capacities of typical mining rigs and do the math. I haven't looked at those numbers in a while, but I would guess 500 USD would buy equipment capable of 1 TH/s. So, it would cost at least 500--1000 million dollars to buy that much hardware.
Then you must add the cost of operating that equipment, which must be around 0.5 million dollars per day.
However, you may instead bribe or buy half of the existing miners. Right now that may cost you half as much, maybe 250 million dollars of capital and 0.25 million dollars per day.
However, if the price of bitcoin were to drop back to $250 or less, much of their equipment would become unprofitable to run. Then you might be able to buy half of it at scrap iron prices.
All these numbers are very rough estimates, of course.