Computing the number of miles driven times a reasonably estimated gas price is a start.
A week in each state is a trip of nearly a year. And because they’re not all neatly arranged in a straight line like beads on a necklace, you’ll have to do some zigzagging and backtracking to hit them all.
That will also take you through some drastic changes of temperature, both geographically and seasonally. Preparing for encountering an 80 or 90-degree range of temperatures will take some forethought.
Are you going to visit any national and state parks that charge an entrance or parking fee?
Are you going to want to stay in a motel now and then just for health and sanity reasons? Where are you going to shower?
Are you going to do any laundry? How about a car wash now and then? Better have a large sack of quarters.
You say you’re going to camp in the car. I hope you’re not planning to park on my street and just sleep there. Will you be needing to build a campfire for cooking? You’ll have to stay someplace where that is allowed. If you don’t prepare your own food and just purchase takeout, food is going to cost more, and you’ll have to plan a route that always bears available services in mind.
You should have an allowance for road emergencies such as a flat tire or a mechanical failure. And don’t rule out medical expenses, whether you scrape your knee on a sandstone cliff or get a toothache from living on sugar-coated cereal.
How about a few regional treats and a souvenir or two?
Are you sending any postcards home?