Considering weather, Haikou located in tropical area must be the best place for staying during winter of Jan. Obviously, these hotel rate will be high during this period. Guangzhou and Shenzhen is next to Haikou, so there is no serious cold air as usual. However, snowstorm has attacked Guangzhou several years ago. Guilin will be wet and somewhat cold, so warm coat and long-johns should be prepared. Chengdu is less wet but cold in Jan, so a downjacket will be helpful.
Ps: the cheap hostels or home inn could be found easily in these cities, all of which are the popular travel destinations.