A good starting point I recommend is reading through the articles at GoodSound (http://www.goodsound.com). If you have trouble parsing the reviewing language you come across, you can try the audio glossary at Head-Fi (http://www.head-fi.org/a/glossary-of-terms) or the one written by the late J. Gordon Holt, founder of Stereophile (http://www.stereophile.com/reference/50/).
SoundStage Hi-Fi is another online audio magazine I like to read (http://www.soundstagehifi.com). For reviews along the home theater angle you can try Secrets of Home Theater and High Fidelity (http://www.hometheaterhifi.com/). There are also the usual suspects - Stereophile, Sound and Vision, The Absolute Sound.
Some more esoteric sites I sometimes frequent (in no particular order):
The Audio Beat (http://www.theaudiobeat.com)
6moons (http://6moons.com/audioreviews/audioreviews.html)
Enjoy the Music (http://www.enjoythemusic.com/magazine)
positive feedback (http://www.positive-feedback.com/)
Confessions of a Part-time Audiophile (http://parttimeaudiophile.com/)
TNT Audio (http://www.tnt-audio.com)
Stereo Times (http://www.stereotimes.com/)
You can also post your questions on the Audio Asylum forum (http://www.audioasylum.com) or the AVS Forums (http://www.avsforum.com).
As far as why components cost what they do, well, the only thing you can do to 'understand' is to listen to some components at different price points and decide for yourself whether or not they're worth the price that's being asked. There is a rough positive correlation between price and performance, but it's by no means absolute (many lower-priced products can sound equally as good, if not better, than more expensive ones). For $3k AUD, you can certainly put together a satisfying system with new or used components.
Have fun!
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