It is around 3600kms from Harbin to Guangzhou. And one train T238/T235 is available on the route by costing about 37hrs. The ticket price for one lower soft sleeper is about CNY1080. Several direct flights are also available on the route, which takes only 5hrs. And some cheap discounted tickets are always on sale. About the details, you can check Cheap China Flights(https://www.travelchinaguide.com/china-flights/) .