Would you be willing to build your own sub box or no? If you are I highly recommend the JBL GT804. I build a sealed box for it, and it sounds amazing, and its only a little 8 inch. Its 60 bucks on amazon right now, and you don't need to go too crazy with the amp, its a 200W RMS sub, so youll be fine with a 300W amp.
If you don't wanna go through all that you can get a sub in a box thats already amped for not much at all. The only experience I have with those is an MTX one thats going for ~$130 on their site right now. MTX Mobile RT8PT Universal 8" Powered Bass Tube
Theres the link for it. Bazooka Tubes also sound fantastic, but I've never heard one of the ones with the built in amp. Those will run you a little bit more too, you can get those that have an amp already too though.