Elemental design has a EHQS12 Subwoofer for $25 and EHQS8 for $15. I don't know what it comes to with shipping though...
You gotta remember shipping subs is not cheap, they are pretty heavy...
Just checked prices with shipping, you would be able to afford the EHQS8 w/c is a 8 inch woofer. Not sure if you would have to pay taxes though, but the sub plus shipping would put you at $28 and some change. Good Luck!!!