Your kid is perfectly normal, and it really doesn't need to be that hard.
First, set her up to succeed. Pour her a reasonable bowl of cereal at night, and put the appropriate amount of milk in a small pitcher of measuring cup in the fridge next to it. We did this for DD when she was 4 so she could make her own breakfast without waking me up :-) but it also provides a pre-set portion. If she is ADHD, like my kid, this kind of planned-in-advance structure is really important, because things like portion size don't come naturally, and impulsivity rules. Do this with any "mandatory" things -- e.g., after she brushes her teeth at night, put the toothpaste on her toothbrush for the next morning so she doesn't use too much, create a specific spot where her hairbrush lives and a routine of when she brushes it, etc.
Second, let natural consequences deal with the rest. It's not even "rationing" -- it's we buy one loaf of bread a week, one bottle of liquid soap a month, or whatever, because that is what we reasonably need, and it is up to all responsible family members to use appropriate amounts to make it last. Since she's a kid, what is "appropriate" may (will) not come naturally, so you might have to show her. But then when she runs out, oh well, here's your backup bar of soap until it's time to buy more.
Finally, expect failure. But so what? Maybe she'd rather have one gigantic bubble bath a month instead of an appropriately-meted portion every day. That's ok. The beauty of natural consequences is that you let her decide what matters most to her -- a giant pile of toast that goes uneaten, or a sandwich every day. And it's not your problem, because your job is solely to buy one loaf of bread a week and one bottle of liquid soap a month, or whatever. Your budget is your budget, your list is your list, but from there it's up to them to figure out how to they want to use what you buy.
FWIW, you are fighting the never-ending parental fight. I can't tell you how many times I heard "shut the refrigerator door, you're letting all the cold air out, you think money grows on trees?" ;-) We all naturally take for granted what is plentiful and free to us. We need a little scarcity (either naturally-imposed or artificial) to appreciate and learn self-control.