That all depends on the room you have to work with and if you want SPL or SQ. 2 $200 subs will play louder, but a good $400 sub can play almost as loud and still sound better. As daniel said, it really comes down to what type of subs, and what type of box.
Yep, totally dependant on what you intend to do. Usually a single $400 sub will do better SQ wise and nearly match in SPL, but it depends on what subs you're talking about. You can get some great subs nowadays for 200 bucks.