^Yes building your own box will probably be cheaper, and over all better, however $100 will probably not get you subs.
Do you already have an aftermarket deck?

__________________
2002 Red Mazda B2300
Short Cab, Short Bed, Pioneer sound system:
DEH-P5100UB deck, dual USB/aux inputs, single CD, organic display, remote
4 TS-A6872R 40 watt nominal speakers
2 TS-SW2501S4 250 watt nominal 10" subwoofers