If you are up to building your own boxes, that Boss sub may be something to check out. I see that the sub is $40 at sonicelectronix.com which is 60% off. OTOH, just doing a quick search on other forums for boss audio yielded several unfavorable reviews. Now you will always find someone who doesn't like a particular product no matter what the brand so take that for what it's worth. $40 is still pretty cheap even if the sub can do half of what the specs say. It could at least be a stepping stone until you can afford something more high end.
Now some brands that I have personally run that I like are Memphis Audio (currently running a pair of 12" Power Reference series which I was able to get locally for $110ea.), JL Audio (highly recommended but can get pricey quick), Phoenix Gold Xmax series 12" (Great tight bass and decent SPL's, but long since discontinued. Ebay may be your friend though) and Kicker Competition series (more of a starter sub but not bad, IMHO).
Brands that I have listened to and liked include the Alpine Type R's, Diamond Audio, Dynaudio (get pricey very quick), Focal (also a pricey sub), Rockford Fosgate (haven't been impressed with their newest offerings, but their older stuff was pretty good, there is a reason they used to be at the top of the subwoofer game.)
You should be able to find a sub from just about any one of these brands that will be within your budget especially if you build your own enclosure, but, in addition to the questions cubdenno asked above, you also have to decide what size sub and how many subs you want. Personally, I think a 12" sub is the best combination of compromises with the whole quantity vs. quality game and typically a 12" doesn't cost much more than a 10". within a given series.
A little more information will be needed to help you narrow down the search even more, but hopefully this gives you a little more information to help you make a decision.