I live in Southern California, and I have a wild caught bimac, which is still small (almost a 3" mantle). I get live food from the beach when it is convenient, but most of the time I feed pieces of thawed frozen shrimp (uncooked! Not freshwater shrimp) or scallop. I feed my octo only every two or three days, and one shrimp is good for at least three feedings. If there are 20 shrmip per pound, three feedings per shrimp, and I feed every other day, then a pound of shrmip will last about four months, and cost about 58 cents per week. I also get frozen scallop, which cost maybe double, so if I feed shrimp half the time, and scallop half the time, then my food cost goes up to 88 cents per week. If my octopus ever gets big enough to eat five times more than it does now, it will cost me about $4.40 per week for food. Obviously experiences vary, but for me, food costs are trivial. I keep my bimac cool (between 60 and 65 degrees) so its metabolism is slowed, and I feed at minimal levels (to try to increase life expectancy), so maybe that's why my food costs are so low (but my electricity costs are high!)
Since you are 13, I suspect that you have a lot more time, and a lot less money than I do, so I suggest that you let Chef Reef teach you what foods are easy to collect and get a few weeks worth of a variety of things. Then cut them into single serving sizes, freeze them separately on a cookie sheet in the freezer (so they won't stick together) and then transfer them to a ziploc bag. That should make it easy to feed for free.