First, not only high-speed train tickets but soft-sleeper, and in some cases, hard-sleeper train tickets can be more expensive than an airplane ticket for the same journey.  It is the law of supply-and-demand.  Train travel is ubiquitous in China and in high demand -- there is no need to discount anything as trains are usually overbooked, to say the least.  The T, K, D, G., etc., trains are particularly state-of-the-art and in some cases city-to-city by G train can be faster than by the airplane, if one considers the check-in time, etc., etc.  Additionally, shipped baggage allowance on the trains in China is quite generous when compared to flying.  Finally, travel by plane, or by Boeing or by Airbus anyway, is a relatively new phenomena in China (meaning within the last twenty years or so) and being a newer market, heavily saturated at that, there is bound to be price-cutting, heavy discounting, etc., just to fill the same planes.  Additionally, many of the airline companies are owned and cross-owned by much larger corporations with seriously deep pockets.

I'd take a high speed train over a airplaine on the same route any day.  D and G trains are more comfortable, convenient and in most cases faster when you consider the transit to airport, waiting for baggage, etc...  Maybe this is why they are sometimes more expensive.

High speed trains are fixed price. Simple as.

If you know anything about flight tickets its that there's a million factors involved in the price.