Not to be an a$$ but frankly, you are asking an open-ended, opinion question. I have a couple of questions for you that may help you drill-down to a success path:
1) You didn't specify what type of head unit you currently have; aftermarket or stock? If stock, then what type?
2) Are you replacing a subwoofer that is currently installed or will you be installing a new component?
3) If a new component, does the $300 include an amplifier and a box for the sub?
And now, here's my opinion: I prefer higher end sub manufacturer's (Focal, Morel, JL Audio, etc.) however, I'm not rich. So, I have to balance my budget with what sounds good. Which brings me to the next point; louder is not always better. Find someting that is within your budget and that you can live with and keep in mind that if you drive with the windows down (like I do), you will loose some of your bass to wind and road noise. The rule of thumb is that for something that sounds twice as loud, you would have to increase the power 4X. For example, if you had a 100 Watt Sub and want it to sound 2X louder, you would have to increase the power to 400 Watts. Kinda puts a damper on the budget, huh?! Just a note but the aformentioned wattage figures are RMS values and not Peak. RMS is a constant value where as Peak is just that...peak.
Conclusion: Determine what you want out of your sound system and buy the best you can afford. Good luck!
P.S. The typical websites I use are
http://www.crutchfield.com/ and
http://www.woofersetc.com/. I like the look of stock but the sound of aftermarket. I use Crutchfield to see what fits and they typically give me dimensions of what fits within the space...I don't buy from them (sorry, Crutchfield). Woofers Etc. has a larger selection of both high and low-end stuff. Do some price shopping as woofers etc. may not have the best prices. Hope it works out for you.
