unit that I have cost $100.00 brand new. I purchased it off of EBay. I can tell you it has made a tremendous difference for my water. If you only have a 30 gallon tank, it will take you a while before you need to change the filters. On average and depending how dirty your tap water is..you replace about once a year.
Also, if you have diatoms real bad they will go away in a week or so. I would also make sure you have a good cleanup crew. I have several snails and dwarf hermit crabs in mine. They take care of any access food, waste, and algae in your tank.