what's "expensive" to you, and how big of a system do you want? deck/speakers/subs/amps? or just deck + speakers?
for a deck, you're lookin to spend under $200 for a good one, under $300 for a good Alpine one that plays MP3s, speakers you're rangin $200 more (i recommend the Alpine Type S's, or any Infinity's)
a good cheap set up for amps/sub would be an infinity sub (90-100) powered by a 2 channel amp bridged to 300-400W RMS (100-200 depending) and wires will run you another 100, the box will be $50-100 depending on DIY or buying it
Deck: $300
Speakers: $200
Sub: $150
Amp: $250
Wires: $100
Box: $75
^^ don't spend more than that on ANY of the items on the list
but if you're goin with a quick system as i just read now lol...a pioneer deck for $150 and pioneer speakers for $100 or so, but get Alpine Type S's, those own ;P