Ditto what other people have said. You can expect to pay $20-30/month on food. Adult cat food is generally unacceptable and is a quick route to a seriously sick ferret. Low/no grain cat food is good, as TTB suggested. I feed mine Innova Evo Cat. It's grain free and 50% protein, 20% fat. Minimal requirements for ferret food is 30% protein, 20% fat. Please do all your researching before getting ferrets. They do need a lot of free roam time and will poop all over the floor and wipe their heinies. I highly suggest getting ferrets from a shelter. If you're near MD, Rocky's is awesome and we frequently have babies in from people that don't know what they're getting themselves into, as well as sweet cuddly adult ferrets that aren't as crazy or bitey as babies.
http://www.rockysferrets.com/ferret_tips/index.htmlwww.ferretvillage.orgThe initial investment in the weasels, the bedding, the cage, the food, toys, tunnels, hammocks, litter..... is a drop in the bucket. It's the out-of-the-blue $800 deposit to leave your ferret at the emergency vet overnight that you need to make sure you can afford. Been there, done that!
For me, I have a Martins R695 ($119+shipping), about a dozen bedding pieces: sleep sacks, hammocks, cubes, floor covers (over $100), Evo Ancestral Cat ($7/3lb bag), Yesterdays News for the corner litterpans ($20 a bag, but lasts forever), Nature's Miracle for cleaning up ferret messes on the carpet ($17/bottle), Hairball Remover, Ferretvite and Ferretone supplements ($10 each), and an average yearly checkup and shots ($75). Weasel kisses on your nose.....priceless.