I recognize the danger of having my goldfish hobby appear on the household expense "radar". So I am always trying to find ways to surreptitiously expand my facilities and activities without incurring any significant expenses. I have been somewhat successful in three areas: food, tanks, and buckets.
For food, my gel food is about half the cost of commercial goldfish foods. So this is good (subject to the caveat in another discussion and the assumption that my time has absolutely no value). The other option is to use commercial aquacultural fish foods. If you can find about 20 people to split a 50 pound bag with, "trout chow" seems to work pretty well and is darned cheap.
For tanks, I have kept a sharp eye out for remodeling and takeovers in the pet "mega-store" chains. Through this approach, Iíve gotten four 55-gallon acrylic tanks at various times for an average cost under $30 each. The best thing about these tanks is that they weigh about 15 pounds when empty, so they are easy to move and store.
The other thing is I am always watching for sales on are large plastic containers suitable for housing goldfish. My metric is 50 cents per gallon or cheaper is a good deal. Sometimes I see a 20 cents per gallon deal, and it is all my wife can do to restrain me from filling a room. One great source of large containers that are potentially free is the 55-gallon plastic drums that car washes use. While we all know it is a no-no to ever use soap in fish containers, it seems to be possible to clean these former soap containers up so they are okay. Most people cut them in half (either crosswise or lengthwise) and use them like that. Koi hobbyists use them whole as drum filters.
Last but not least: 5-gallon buckets. I use these for changing water, treating sick fish, building filters, storing pond plants over the winter, Ö you get the idea. The trick here is to strike up a relationship with your favorite restaurant (for me, small ethnic eateries have been the best sources). Most of these places receive bulk ingredients and condiments in these containers. They usually throw out a couple a week, so an extra dollar on a meal tip each week can bring a potential steady stream of these useful containers.