how cheap? you can get a plastic bucket cut o hole in the bottom, fill it with media. put it above the tank, pump water in at the top and let it trickle back into the tank.
thats cheap.
internal filter.
a big block of sponge inside the tank. get a short length of tube push it half way threw the center put a power head on the other end. cheap power filter.
if you want to get more technical there are a few ideas knocking about.
how about a smaller plastic tank above your tank say on a shelf, drill a hole and fit an overflow with a pipe leading back to your main tank. you could have a over head sump with no need to drill your glass tank.
