The most expensive way to travel…
is now to travel by bus.
Before, a ticket from Haikou to Shenzhen would cost 260RMB, and a ticket to Hong Kong would cost 291RMB. Now a ticket to Hong Kong costs 361RMB. Even a bus ticket to Guangzhou costs 261RMB. 281RMB if you want a sleeper bus.
To put it another way, it is cheaper and faster to fly from Haikou to Shenzhen. Even when accounting for the bus to/from the airport on both ends of the trip.
The cheapest way apparently is the train. I found a travel agent that has student discount tickets for the train. If I can get a seat (not a sleeper berth), I can get to Guangzhou for 70RMB. The only annoyance is that you can’t book a train ticket more than 5 days before the train departs, and you have to pay a 5RMB booking fee. Well, there is also the fact that the train station takes a looooooooooong time to get to in Haikou. But even taking a taxi, it is going to be cheaper than taking the long distance bus. Not to mention far more comfortable. Train sleeper berths are flat, and are long enough to accomodate my body. They also allow luggage storage within my line of sight.
Update 2007/3/21: After asking around, I found out student tickets are only available during summer and/or winter vacation.