Simple - do NOT plan on doing it all at once

Let's think - water-shelter-food

You drink Soda? Know anyone who does? (if you don't even know someone who does...) Get some of the empty 2l bottles. Clean them, and you can store water in them - cost - about 30 cents/person (2 bottles/day/person)

Shelter - I'll leave that up to you

Food - You DON'T need MREs or the like. Most folks will be "shelter in place" or at a shelter. Think about canned foods you LIKE to eat. Soup? That's good. Instant Soup? Good. Ramen Noodles - heck, at 10 cents a pack, even if you DON'T regularly eat them....

The trick here is 1)Packaging and 2)Buy what you eat, eat what you buy. My wife loves tuna fish - I like tuna fish - my kids will eat tuna fish. My wife eats about 1 can/week. It's no real hardship, when Tuna goes on sale, to buy, say, 6 cans - and when we get down to 3, buy 3 more (or 6 if you are trying to build your supplies) - that way, there are always between 3 and 6 cans of Tuna in the house. I like soup - so I bought 8 or 9 cans - when we get down to 4-5, I buy more. The trick is, do NOT plan on buying it all at once. I keep some food in a go bag, but it's not necessary - I can walk by the pantry or cabinet in the kitchen and grab a LOT of food, fast

First aid stuff? You keep bandaides, etc in the house? Put them in the "Bag" instead of the medicine cabinet - now you walk to the bag if you cut yourself, and besides, if someone gets hurt, the first aid stuff is in one easy carry package

Think about it - you probably have most of the stuff for a 72 hr kit in the house RIGHT NOW, just stored in different places - so you just store the stuff in the kit - just remember to replace it as you use it