Main developer of Litecoin Charlie Lee, alerted crypto users how easy and cheap can be attack of some proof-of-work cryptocurrencies. For example one hour 51% attack on Bitcoin costs $553,982, on Bytecoin attack costs only $557. This list is only theoretical, because if somebody would invest to attack coins like Bitcoin or Ethereum, it will costs a lot of millions, but after attack all media will inform about this attack and probably there will be fork to the state before attack. Which can make this attempt very expensive and useless.
List of 51% attack costs on PoW cryptoccurencies