Well depends on how big you want your box. I just built a ported box for 1 hdca 12" with 2.78 cubes of space. Not sure on port area, around .875 cubes. I got 3/4" mdf wood, 4'x8' sheets for $30 a sheet at lowes (You may need two). Then you need a screwdriver (preferably a drill). Screws are around $5. Then you need caulk about $4-$10. And a caulk gun, which start about $10-$30, but I found mine quite a ways over for $2 (made in china, but it gets the job done). Then if you paint it it'l be about $7 or so, I would liquid rubber (rhino liner it) or get actual wood paint $10 a can, and a brush and paint it. Not sure on carpet, but its cheap. Now when you put it together I think you need to drill tiny holes first, because I screwed the screws straight in and it split the wood bad.
So without paint/carpet, your looking at about $50, unless you dont have a drill or screw driver