Filter material substitutes

I have just purchased an Eheim Liberty, hang-on
filter.(smallest one). I can predict that, over the life of this
filter, replacing the sponges and the carbon-infused whatsit,
regularly, is going to run me into big bucks.The "biofilter
cartridges" alone are $3.50 each,Canadian.

Rather than being locked into replacing these with Eheim
parts, has anyone had any luck or suggestion for, devising cheaper,
but effective filtering media that would work as well as the Eheim
inserts?

Thanks for any information.
Blair.