The majority of modern PCI sound cards made in the past two or three years will have a signal-to-noise ratio and frequency response range high enough for all but the most demanding professional applications. So as far as sound quality is concerned, a $10 card today will sound just as good as a SoundBlaster Live!
The real reason the Live costs $50 is its feature list: 3D positional audio with EAX and A3D. Many soundcards today are taking the winmodem approach, and offloading some processing onto the CPU, but the SoundBlaster Live has its own DSP which can handle most of the work. Even so, the cheaper cards are a much better value, as many of them can emulate EAX and A3D with decent CPU utilization.
Oh, and you shold ignore any spec dealing with "amount of simultaneous MIDI voices", for instance, SB PCI 64, PCI128, etc. The fact is that MIDI's are all but obsolete for today's applications, and they never used more than 16 voices to begin with.
Modus