Its always cheaper to build your own box too. And not that hard if you can handle tools. the pioneer 306c mentioned above is a good sub and if you build a ported box ~2cuft tuned to 30hz it should be awesome. Then a jaycar two channel and your laughing, should be about 500 with power cable etc but you'll need to change HU probably.
Im sure youve already made your way onto CAA based on the recommendations of others ^, so head on over and investigate box building and save yourself minimum 50 bucks and get vastly improved performance.