Only 300K needed to perform a 51% Attack on Bitcoin?

Blockchains are known for their transparency and immutability, but is it really that robust? If a group of miners control more than 50% of a blockchain network’s mining hashrate, or computing power then they could do some serious harm to that blockchain (so-called 51% attack). At Bitcoin Wednesday I talk with Martin Kuvandzhiev, Co-Founder of the notorious Bitcoin Gold cryptocurrency. How much does it cost to perform a 51% attack? Some say $300k/hour others say €1.4 billion for Bitcoin.. What is true? And what harm can it do? How can networks defend themselves? Martin talks solutions!