Hey,
Copenhagen is absolutely right, here's why better cd's cost more:
When you play a patch on something like a Triton, there is an awful lot of programming that has gone into making it sound so good. You not only have the basic waveform, but there are also parameter settings for the envelopes/LFO's, amps and filters and DSP effects that are being used.
For more complex patches like pads or orchestras, there may be multiple waveforms used; taking into consideration that each waveform has its own set of parameters, it becomes mind-boggling to think of all the minute details put into making the patch not only sound good, but also responsive to velocity, aftertouch, key scaling, mod sources like pitch/mod wheel, etc.
Programming these patches takes an awful long time, especially if you're starting from scratch; making them sound good takes even longer. Doing a patch for something like an MPC is relatively easy as it has a simple architecture; doing it for something more complex like an Emu sampler takes even longer because there are more possibilities. Akai and Emu aren't always compatible with each other, so programmers often have to program patches for both formats; this takes twice as long.
When you buy one of these cheap sample cd's of the internet, what you usually get is just a cd recording of someone playing the patches; you don't get the original waveforms, you don't get the parameter programming, you don't get the effects. While this is a quick and dirty way to build up your sample library, it really sucks hard when you want to edit the patch...
Take care,
Nick