Funkywaves won't have anything that cheap, and plus shipping alone would eat up most of your budget if you live in the US.
If you can push your budget, my first choice by a long shot would be the Outlaw Audio LFM-1 Plus, which is on sale now shipping for $467. That is an outstanding deal for that caliber of subwoofer, but that deal only lasts until the 30th, so don't wait too long on that one. Another excellent sub that still pushes your budget but a bit less would be the Emotiva Ultra Sub 12, which is $429 shipped currently. Another couple of good subs that exceed your budget a bit are the Elemental Designs A3-200 and A3-250, which cost $415 and $450 respectively.
If you absolutely cannot exceed your $400 budget and would really prefer to spend less, I would look at the Outlaw Audio LFM-1 Compact, for $339 shipped. That would be a good little sub, I used to have its sister Hsu sub, the STF2. But if you want the best bang for your buck, like I said above, as far as I am concerned, the best value from all of these would be the LFM-1 Plus.
