Power supply for the server should be of better quality in comparison to the PC PSUs. Ordinary people simply do not use their PC day and night. Plus, I do not think that 2400W PC PSUs exist. While we can easily find so powerful power supply - http://hardware.nl/power-supply/hp/jc747-61001.html
Regarding the price difference: you only need to compare several values. For example, the weight of the PSU you are talking about can be much higher. It means that capacitors and chokes are bigger and are made from a better materials. Additionally, the PSU may endure peak loads for more time without failures.