Sanya is sort of a beach resort, and as such, overpriced.  On Chinese holidays (like Spring Festival) it is really overpriced, but during slow months it is possible to get a few bargains, like a package with round trip flying included.  Last summer I got a week for two with hotel accomodation and free breakfast included for just under 6,000 yuan, hotel on the beach, very nice room.

Just read news today that 75% hotels have been booked during the Spring Festival.  Many tour groups from Russia, Korea and expats living and working in China.  It is indeed a great winter escate destination.

Hainan is really great.  I am trying to understand HappyExPat's post above -- did the week for two include airfare for two, hotel accommodation and breakfast (usually standard)?  If it were for two, then it strikes me as reasonable.  If airfare were not included, then it strikes me a tad expensive.  Try www.elong.net.  There are a  good number of hotels still available in Sanya and surrounding areas with nice digs and not that expensive either.