I found a Tetra Diatomagic filter going for $25 barely used in my local online classifieds. If the filter could use any diatomaceous powder, it would have been a deal - not otherwise because it's discontinued. Anyhow, look for deals like that maybe? HOT Magnums do go cheap sometimes used (or even new) on AquaBid and such, and that micron cartridge does the job better than any filter floss ever could.
I would still say to get a canister filter though, you're going to need massive amounts of bio filtration when these fish get full grown. And I'm with most everyone else here, there's no real need to spend extra on premium canisters like Eheims and Marineland when Renas work just as well. I've even had good luck with Penn Plax canisters and they are cheaper yet.
Where you should really spend the money is in the media - I'm a big fan of Seachem's Matrix (the biological media, not the carbon)