I have been creating my own filters for quite some time now, and ive learned a lot. The one I currently use is pretty basic, and you can use whichever filter media youre comfortable with. The thing I like is its outside the tank and the pump never gets clogged up. Below is a pic of the whole setup as it is now. Its a 55 gallon, the filter container is just a rubbermaid propped up for now. Eventually I will use a 10 gallon tank for the filter, but I have to build a stand that will support it. Basically, the large plastic tube is nothing more than a siphon, the larger the better. Once the water is pumped into the filter container, it passes thru whatever media you use. I have lava rock, some aquatic plants, some fine plastic mesh, and activated carbon. after the water has been filtered there is a small submersible pump at the other end that pumps water back into the tank. you want the siphon going in to be larger than the pumped water so the water level stays the same in the tank and the filter. If the pump ever stops, the siphon will stop too. This idea Ive just come up with recently, and although it is the best filter ive made yet, I still have much to improve on. the cost is basically a submersible pump, some plastic pipe, a rubbermaid container (or small aquarium), and filter media.
the link URL is a picture of how it looked the first time I built it.

http://www.iturnrocks.com/images/filter/filter0303.JPG