There's a website that has javascript speakerbox calculators. It doesn't get much easier than that. Here it is:
http://www.carstereo.com/help2/Articles.cfm?id=32
Fortunately, low budget and serious club bass go well together. The trick is to use a ported box, with the tuned frequency at around 40Hz or so. By giving up the lowest octave of frequencies, you gain a LOT of volume with very little amplifier power. You can also use efficient speakers that don't have much excursion.
The only catch is this: building a ported box for a speaker must be done right. If you don't have the specifications for the speakers you salvaged, you need a lot of luck and trial and error to build and properly tune a good ported box. If you don't build it right, it'll have that horrible one note wet-sounding thump you hear from passing cars spraypainted with grey primer.
If you can shell out a few bucks, there are cheap drivers aplenty to be had from MCM and PartsExpress, starting at $12 and going up from there. And getting the club bass slam can be done with relatively inexpensive drivers with a proper design.
Good luck!
