with audio, you get what you pay for. (thats my opinion anyways).
If you want to save some money, do some research and try and buy everything from the same store all at once. Most stores will do a deal if you buy everything from them, and do a good deal on installation. If you got a mate who works there, you can always get a little more knocked off the price
and uh, remember it costs to have it all wired up. No worries if you can do it all yourself but if your not... auto-electricians are expensive!
thats my two cents
p.s
the brand name things are more expensive... you may pay a little extra for the brand. But remember, theres a reason why they are so popular. And also the warranty is usually better with a popular brand and more easily replaceable if it does break