this only work to save cost if you go big scale to breed to feed your monsters. usually the fry are too small and with the amount of food you will need to raise it up, it would still be cheaper to buy direct from LFS plus the time and the space you will have to use to set up these tanks.
Well that's not a can do attitude

All kidding aside, you are right, the only way to be practical is to go large scale if you are feeding true monsters. If that is the case, then an outdoor feeder pond may be in store. To be cost effective, you either have to have a source of free food or buy in bulk such as pond fish food. $3 for an ounce of premium flake food will get you no where if you are trying to save money. You can buy ten pounds of pond food for $18 at Petsmart and if you really go large scale, you can buy 50 pounds at a time on eBay for more savings.
At the moment, I am feeding with tadpoles. They are completely free and all I had to do was not add chlorine to the swimming pool after the tadpole fairy brought me some frog eggs. I have four ten gallon containers of them and they are morphing faster than I can feed them to my fish.
As for the fry being too small when they are born...well you can't expect adult fish to come out of the momma fish. You have to have patients to let the fry grow to feeding size. It is not realistic to think that you can start breeding this week and to start feeding next week. Additionally, you should not expect what comes out of one or two guppies to satisfy a monster that eats a dozen guppies at a time. If your monster requires medium size goldfish, then it should be understandable that you will have to have a very large breeding operation if you want to raise your own.
best cost saving method is to get your monsters off live food.
That wouldn't be any fun now would it? Besides, many fish will only eat live food. Feeders, when done properly, are a food delivery system. You should keep your feeders in a tank for a day or two for 'gut loading' with pellets or flake.