It didn't look sold out when I was there. I had bought my tickets way in advance at full price on the F1 website since I wanted to pick my section and I wasn't sure if it would sell out or not. Mind you, even at full price my tickets were only about $100 or so for decent seats near the hairpin turn. China is the cheapest of all the races.
The most expensive seats were about $400 for the 3 days. There was also some "early-bird discount" on the F1 website when the tickets went on sale.
I would hate to go there and then not be able to score tickets.
There were also about a million scalpers at the track but I just can't trust that the tickets they were selling were not fakes.