In order to calculate how much money do you need to perform a 51% attack on bitcoin:

Current hashrate (Total network hashrate): 36349358,259 TH/S (From https://blockchain.info/stats)

We choose our miners from here:
https://en.bitcoin.it/wiki/Mining_hardware_comparison

Let's take for example: Antminer S9
Hashrate = 14 TH/S
Consumption = 1.375 kW

Cost of harwares:
Hardware = Current hashrate * Price of Antminer S9 / Hash produced by Antminer (per seconds)
Hardware = 36349358,259 * 2400$ / 14
Hardware ~=  6,231,318,559 $

Number of antminers = Hardware / 2400 ~= 2,596,382

Cost of electricity:
Electricity consumption per day = Number of antminers * consumption * 24
Electricity consumption per day = 2,596,382 * 1.375 * 24
Electricity consumption per day ~= 85,680,606 kWh

Electricity cost per day = Electricity consumption per day * Cost per 1kWh
Electricity cost per day = 85,680,606 * 0.045$ (Electricity pricing - Depend on countries)

So attacker have to pay 3,855,627 $ of electricity per day !

This without make calculation of transport, renting or buying local where to install all this stuff, without counting the difficulty to get this number of miners and electricity power required to get them working all.

And counting the risk that the attack will not be a success and many other factors ...

If you want to know more about Difficulty and Hashrate: https://en.bitcoin.it/wiki/Difficulty

I made calculation 2 days ago from here:
https://bitcointalk.org/index.php?topic=4429114.msg39597354#msg39597354

In the space of two days, we can see this changes:
Hardwares:
Today = 6,231,318,559 $
2Days ago = 5,936,594,032 $

Difference ~= 294,724,527$
Additional AntMiners ~= 122,802

Electricity:
Today = 3,855,627$
2Days ago = 3,673,266$

Difference ~= 182,361$

In the current state of things, it's near impossible to attack Bitcoin ! This show how this coin is strong now.