First, what audio system do you have? if you add a sub, you will need power, so you will need a AMP as well. If you are replacing a sub, then you already have an Amp. Does the Jeep radio have built in crossover filter to feed an Amp signal? I assume so... Third, you will need an "input" line to run from the amp to the subwoofer.
Where do you want to put the sub? You will need an enclosure and if you want to put the sub in the side compartments in the back, you will need a sub that is really narrow.
Madness autoworks sells a sub with enclosure that is designed to fit into the rear side space...but it is not in your budget- 450 dollars and you will still need an Amp.