You should just get an Alpine MRP-M400 (450? I forgot what number) It's a good 400 watt amp and retails for $200 usd. It will really make the bass hit cleaner and deeper, not to mention louder, if you choose. Definately worth it. Plus I didn't know you could even power a subwoofer from the head unit's amplifier those things are usually like 50 watts or so and most big subs take 250w+.
Check ebay I'm sure you can get a good deal on an amp, but I would reccomend staying with a big brand like alpine, kicker etc.