Cheapest I've ever found come from BKelectronics.
the sell drivers as simple plain drivers, (and also sell all the fittings to go around them.
Their cheapest speakers are the skytec range, (as used in jpleisure equipment).
I don't think their full range is displayed on the website, so you should enquire about a catalogue, (and whether they ship to the US!)
Here are some recomendations.
Firstly. don't buy, build, that way you can get beter quality drivers, (and in many cases a better quality sub as well.) this is an especially good idea if you want to save a few quid, or want to make sure that you have the best quality speakers.
Secondly, when looking for bass speakers, 15" subs are a waste in a car, go with 12", the frequency response is usually beter anyway,
(there are some nice eminance speakers on the bkelec site for £25 these would be ideal for a car).
if your have a solid parcel shelf, (which you do) and there is enough space to mount the speakers, I'd suggest that you simply cut out holes in your parcel shelf and mount the speakers there. that way you will hear the bass as well as feel it. -though make sure that your parcel shelf is truely solid enough, if you weaken it too much then the acoustics will sound bad, and it'll reduce the effective power out...
the pros of mounting bass speakers in the parcel shelf is that it effectivly turns the whole boot space into a bass box... -the bad side is, every time you put stuff in the boot you alter the acoustic propeties of your bass box...
Failing that, build a box to go in your boot from Think, (3/4") MDF wood, build it in sections with a proper wooden frame, and don't be scared about how much glue/how many screws you use, the better the construction, the better the sound quality.
if you are building your own sub, then you might want to search for a bass port calculator, that will tell you how bigger hole you have to cut to allow air movement, a sealed cab is inefficient because it compresses/decompresses limiting diver movement.
when/if you get a catalogue, you'll find that they sell speakers that are bass speakers with mid/high range speakers actually mounted on the bas cone... -don't buy these, they look and sound crap.
for your mid range you'll probably want to look at some 6" or 8" speakers, these will provide ample power and a massive high range frequency response.
again from the BKelec site.
the eminance alpha range seems to be a good way to go.
Mount these in the doors. either two front doors, (or if possible get four, then you can have front and back doors).
As for amplifiers.
All I have to say is anything over 500W is a waste, my car steros have a 70W head amp built into the stero, and you can hear that down the street when and if I have it turned up. (that's with the windows closed).
Ideally, look for something that is just rated at 1000W, (this is probably 500 per side), also try to make sure that this is RMS not PMPO, (rms - root mean squared, PMPO - peak maximum power output).
a 1000 RMS amp is more powerful than a 1000 PMPO amplifier.
When you are setting this up you'll want to get a good crossover unit, a cross over filters the signal, to stop low range going to your high range, and to stop high range going to your low range drivers.
high end frequencies going to a low range driver decrease it's efficieny, since there is more impedance to high range signals., the same, (but reverse for low range going to high range).
Ideally, get a head unit (stero) that isn't amplified, and sends out a line level, then use line level crossovers to filter the signals,
send your high ranges to one and and send your bass to a dedicated bass amp.
of course, now you've bough 6 speakers, two crossovers (left and right) and two amplifiers, (high and bass)...
it'll have gotten expensive,
If you want to build in stages, try to get a head unit that can give either an amplified signal or a line level signal, then put amplified signal crossovers, this way you can get the speakers / stereo and most of the wiring in, and running off the head unit, before you go out spending money on a high end amplifier...
if you want 500 watts of power, buy a 700w amplifier.
if you want 1000W buy 1500W.
there is nothing worse that hearing someones stero sound horrible and distorted because their amplifier is clipping because it doesn't have enough head room...
and when you are buying speakers, make sure you buy ones that can handle as much power as you intend to put into them. (even if you are buying the amps at a later date).
and when buying an amplifier, make sure that you are not conned by a shiny box proudly declaring 500W, this is most likely 250 per side PMPO...
so (as far as I'm concerned that makes it a 125W amplifier, (~125 per side RMS).)
Hope that helps.