The most cost effective way is to buy good used equipment and/or online/ebay etc...same goes for install. If you know what you're doing or have friends that can help you out, doing it yourself is the most cost effective way to install your equipment.
You may also want to use 6x9 component speakers that are direct fit replacemnets for your stock speakers, such as these MB Quart units:
MB QUART - a Brand of MAXXSONICS - <b>RVF 269 </b> 6x9" <nobr><img src="/grafen/neu.gif" border="0">- ENGLISH
some might say that a 6.5" will sound better, but quite frankly 6X9's of good quality sound excellent, are louder than 6.5's, and easily handle and deliver lower bass and more midbass than a comparable 6.5. Something to consider since your car is set up for it already. If I was outfitting that car, I'd use 2 sets of the Quarts, with a 4x150 watt amp, and a 1000-1200 watts for the subs...1600 watts of loud, clean, and sounds great.