You say you want a big sub, but what kind of bass do you want? Low and boomy (bigger subs i.e. 15") or sharp and punchy (smaller subs i.e.10")
Word of advice though....don't buy really cheap subs for like £30, even if they are rated at 10,000,000 watts, they'll sound poo.
Like Aceritz said, get the best you can afford (but not Sony)