With a Scion tC you should be fine with a single 10" or 12" as it is a pretty small car, you don't want to take up too much trunk space. I have a Mazda Protege, aka pretty small, and I have two 12" Rockfords with a 225 amp and closed box and I can't even get my skis through the backseat from the trunk it's so big, at least the bass is unreal (imagine setting a glass of water on the trunk and having it dumped on the ground by the first hit). Also watch out for the wire kit, lots of people forget that they have to purchase that even if they are getting the subs professionally installed, that will set you back around $100. Then on top of that most installers charge about $44 dollars per speaker plus a nasty flat rate for the amp. It sucks. My best advice, troll ebay and craigslist for systems, often times the sellers still have them in their cars so you can test them out before you buy them, and if you get lucky they will throw the wire kit in too. If you don't want to risk a used system, I'd keep your eyes open for an all-in-one deal. I know that Rockford sells a complete set (single sub, mono amp, etc.) those will probably be close to your price range. Also, make sure to check that your deck can support all the wires you need, you'll need at least one RCA plug-in so you might want to invest in a solid deck.
I hope this helped you get a little beter picture.
Good luck dawg, it's a tough investment but when you finally get it I promise it's worth it. People turn around to look at me before I even pass them and I know that's exactly what you are looking for! Plus it's nice to have an unaware chick in the front seat and then BOOM...clothes get blown off.