Marineland and Exo-Terra brands are the least spendy canisters. I would recommend a couple of sponge filters if your aim is to aerate and filter the water. If you're interested in saving money on filters and media, look at the sponge filters from Aquarium Solutions. I use the Bacto-Surge sponges and they work fine. They're about $15.00 shipped. Even with the cost of an air pump to run the filters, it's still cheaper and the sponges will work as well as anything.
"Fear not, my young apprentice. Just change the tank water."