I used about 1/4 of a 4x8 sheet of 3/4" MDF which is about $40 for the whole sheet.
I then used some wood glue...$10? Some 1.5" wood screws...$5 for a box of 100? A speaker terminal Amazon.com: ATLONA SPEAKER TERMINAL BOARD AT100052 Atlona Technologies: Electronics about $5, and I still need to buy some carpet. Home depot sells in/outdoor rated black carpet that's a really good match for the stock carpet (and is what sonicHD used above I believe) for $3.60/ft and comes off a 6ft wide roll. I haven't figured out how much I'll need yet but I don't think that's going to be a lot.
So if you don't have any of those materials but you do have the tools to do it (rip saw, jig saw, drill, drill bits, screwdriver bit, measuring tape, pencil ;)) it should cost about $50-$60 to build it, then stuff whatever smaller than 12" speaker you want in it...