It seems to vary a lot. My Bimac is happy to eat pieces of thawed frozen shrimp and scallop, so food costs me almost nothing, but some people need to feed live food, which can cost a lot (like crabs from a fish/aquarium store).
Also, my bimac needs a cold tank, with a chiller, which was expensive to buy, and uses a lot of electricity. If your octo eats frozen food and you don't need a chiller, I think it is cheap, but a lot of people seem to spend a lot on food.
Since I maintain 8 saltwater aquariums (and usually a pico or experimental tank or two) it is hard to break out just the octo costs. I spend roughly $50 a month to feed both my briareus and macropus (macropus will not eat frozen shrimp so she gets live fiddlers and clams, briareus gets frozen shrimp most days and live fiddlers once or twice a week). I change out 20+ (5 a week) gallons of water on each tank monthly, replace the carbon (about half a cup) monthly and top off freshwater daily with home filtered RO/DI water. I keep a few soft corals and serpents in their tanks and supplement with frozen Cyclop-eeze and frozen mysis daily (adding another $10 to $15 to my monthly food bill but the frozen food expense includes feeding the other 6+ tanks as well).
Tank costs seem to be running between $1,000 and $1,500 (see tank talk forums) but can be set up for less if you are willing to go with used equipment, do a lot of DIY and wait for exceptional pricing to show up on eBay or Craig's List.