My room is about 15x20. Is that considered medium or large?
Is that dayton including shipping? Would my room need 2? When you use 2 subs, do you put them near each other or on opposite sides of the room or does it not matter? I really need to watch my budget.
I would consider your room medium on the large size of medium.
Because subs have low frequency and the wave length is fairly long, on good way to place the sub is to walk it around the room. In otherwords, try different locations that are available and see which one sounds best.
If I were you, on a budget, Iwould buy one Dayton and see if that satisfies you ( for right now

after all audiophiles always upgrade
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).
You could always add a second Dayton. By the way with multiple subs, the best sound usually comes when they are matched, i..e the same size and brand of subs.
Now for cost of Dayton, from Parts Express the cost is $155.23 plus $14.95 shipping ( subs are usually pretty heavy thus the shipping cost, a lot of Parts Express orders ship free).
http://www.parts-express.com/pe/pshowdetl.cfm?=&PartNumber=300-635&gclid=CJ7Ykafc4JwCFRwhDQod4hR8uQ
Good Luck!