[CBFryman]

$100 will get you some decent Coaxils and $150-$200 can get you some good componets. how much bass are we talking about adding? to match factory speakers you can be more than satisfied for less than 400 easyily. if you are looking to seriously over power your factory stuff and eventually upgrade the front stage 650 can get you goin'
My Reccomendation if you are just looking to mach factory out put in the low end is Reaudio RE's or a shiva, or and atlas 12 or some of IA's lower line, or some of DD's lower line.
if you are looking to seriously over power it.
Koda or New Brahma or SX or Atlas 15 or Avalanche 15 or DD middle or ID....
for power it is woofer dependant but hifonics BX 1500 or 1800 will do well on pretty much all of thoes (some over power some under in a ported enclosure) the most expensive on there is the new brhama i beleive (i think adire is changing the price, they should) and the cheapest Image dynamics or atlas 15. really IMO atlas 15 ported with Hifonics BX1800D will not only well over power factory but it can be had for under $500.
atlas 15 $169
Hionics BX1800D $200 (Ebay)-$300 (authorized dealer/sounddomain price mach)
so tops $469
then $50 for an enclosures more if someone else builds it...but since yo uhave a buddy that hopefully knows what he is doing $50.
then the rest you can add to the $100 to replace factory speakers and get an RE componet set or Ascendant componet set if they ever come out with one and a 50wx2@4ohm amp or there abouts and you will have a pretty nice system.