How much money would it take to bribe the majority of miners to attack the system?
In a parallel thread on "https://bitcointalk.org/index.php?topic=2512771.0" I didn't do the exact math, but a rough calculation shows this:

Currently there is a hashing power of the equivalent to roughly 800.000 mining boxes type "AntMiner S9". For a 51% attack you'd need to buy 400.000 + 1, and bring them on the network (their power cosumption is 1200W, so you'd need a coal or gas power plant, industrial grade level). If you want to "buy" the miners (bribe), then it requires a lot of communication to them, and a financial incentive. Would the miners follow the fork (bribe), if they can see, that they will loose the value of the bitcoins to be mined? I mean, after such an attack would have worked out, the value of bitcoin would run against zero (quickly). What is the remaining incentive for a miner?
Your one-time bribe would maybe need to be equivalent to maybe a 1 year income of a miner, and this for 51% of the miners. Currently a 12,5BTC reward is ~10.000 EURO/CHF/USD, 6 per hour, 21 per day is 18mio a day. And you need 51% of miners. Big gee ...