They could simply buy 3 billion worth of equipment (resell price) so way cheaper produced and with that 30% added they would force a ton of other miners in bankruptcy, taking over the hashrate pie, so rather than adding 600exahash to subdue the current one they could simply replace them by bankrupting and making mining less profitable for other miners, while still making some money.
This is a good point. They could also cause a death spiral by getting 51% for just a few days causing bitcoin price to tank and miners to go bankrupt much faster and making it easier to maintain the attackers >51% hashrate. So might only require ~100mil worth of total power to end bitcoin. The limiting factor in both cases seems to be the ASIC miners but china has a monopoly on that.
Im not sure if China would have to build additional power plants for this or not. If so, that would greatly increases the cost to pull this off. They would need 12-20GW of power and I think China is capable of producing ~900GW as a whole country.