Hard to save money on a DIY sub considering the simplicity of design and cost of shipping. Lots of one note boom boxes out there, many of which don't go into what I'd consider subwoofer frequency range.
40 years ago I built a pair of DIY passive bass boxes: 8 inch drivers in 6 cubic foot transmission lines. Wow! Musical, fast, and powerful. Rated at 114 dB at 17 Hz. But cost $500/pair (drivers, assembled crossovers, instructions, and foam) and came in a 16 cubic foot box.
