"Hey, we'll buy half of your Bitcoins for $1 billion (which is a crappy price), but in return we'll officially safe guard Bitcoin as a currency."
If you were the Winklevoss, would you take the deal?
The Chinese government is pretty crafty, they tend not to pay over 100x the market price for things.
It's not that simple. The market price for 1BTC may be $145 or whatever, but the market price for 500,000 BTC is far higher - because there just aren't that many offers on the world's order books. If you vacuum up the entire book, and then vacuum up all the offers that appear while you're doing that ... then the theoretical price is infinity since right now nobody is offering that much.
If they did indeed want to buy the entire Winklevoss stash, then the Winklevii could certainly reasonably demand a number far higher than the current market price.
This issue is called liquidity, and it's the reason why there aren't more very rich people piling into bitcoin yet (surely some of them see its potential).