I don't buy cartridges every month, I just swish them around in the old tank water when I do a water change. They last like that, you don't need to replace them monthly, you can if you want though.
I also have my own filter media which is cheaper, I use AquaClear foam and cut it up to fit into the filter, then I use activated carbon from Wal-mart and tie it into some clean pantyhose and then the carbon floss is never replaced so I save lots of money on that.
But once a tank is set up, I don't spend too much money on it. My NLS pellet food has lasted me a long time, I paid 4 dollars something for it and I've had it 6 months. I open it and it looks like I've hardly touched it and that's feeding 20 fish nearly everyday with 5-6 pellets for the youngin's (I've got two males who are prone to constipation so they get 2-3 pellets a day depending on if they've pooped