16,946,489 terrahashes per second right now for the BTC network. According to some website I found you can get an ASIC that does 4.73 Terrahashes for $500. That means you'd need 17638182 TH/s at least to get 51% hashpower. That means you'd have to buy 3,729,003 of those $500 miners which would cost $1,864,501,313.80. Let's double that and call it 3.6 billion for facilities and energy costs. So yes, totally possible for a government to create a cluster with 51% of network power if they chose to. I imagine there would be pushback against spending billions to destroy people'a assets though.