the only "cheap" way is to build it yourself and don't expect prefab prices for a nice custom built box for your sub/vehicle. Most on here seem to have very reasonable prices from what I've seen, so get quotes from a few guys...
The cheapest way to do it is to go to lowes and buy a sheet of 49x98 mdf for $37.99. Then find some friends with power tools and clamps and glue, which can also be picked up for $4. Right there thats $42.99.
I only spent the money on MDF because I already have everything else.