First off, I'll say this. I'm not saying this stuff to bash you, I'm just awfully confussed.
You say your mum spends $40 a month? Thats insane. Right now, we're feeding four dogs. Skylar eats like crazy, and he is a big dog (probably bigger than Roxy). He's over 50 lbs and growing. We buy our dogfood from a wholesale store, and we only spend around $30. That lasts us over a month. With your dogs, it should last around two months (seeing as you have 2 dogs and we have 4.)
Maybe go to a store that sells stuff in bulk, and buy enough to last you longer. Usually if you buy it in bigger packages, you save money in the long run.
And I agree with whats been said. I know you want to "help" the bettas in need, but they'll get homes. Just because you don't "save" them doesn't automatically mean they'll die a slow horrible death. I know I don't have too much room to talk, since I've taken in bettas in need, but lets keep in mind that I have enough money to pay for food for every single one of my rodents and the cats. Hey, you're just one year younger than me, I believe, so you can make enough money also. If you have to, have a yard sale or something. I'm sure you have alot of junk laying around that you don't need anymore (we all do!).

Part of being a pet owner is having to sacrifice. I say stop buying most anything for yourself unless you absolutely need it (and that includes bettas). Save your money. If you don't have enough money to feed your dogs good food, then that means you wouldn't have enough money to pay vet bills if your dog got some kind of life-threatening illness.
Anyway...
Good luck with your dogs^^!