Each 10" standard housing was around $30. The model I use is GE GXWH20F. I preferred the clear housing to be able to view when to change each cartridge. There is a 3/4" x 4" schedule 80 pipe connecting the two units. Both the input and output are schedule 80 3/4" threaded, 3/4" hose.

You can find the 10" cartridges on Ebay in bulk for $1 a piece. I change the cartridges every two weeks. You can probably find the housing cheaper on Ebay than what I initially paid also.

Due to the design I'm not sure you could use bio balls. Water comes in from the outer portion and is forced through the cartridge into the center and then out.