Travelling the Main Street of America
To give you an estimate for your fuel and food budget based on actual, achieved results.
From my arrival in at O'Hare Airport, Chicago, to the end of my exploration of the Monterey area, I drove my Chevrolet Malibu hire car a total of 3268 miles. This included a huge amount of local commuting in various towns, and back-tracking to follow up leads and try alternative routes. I would see this as being the absolute maximum that anyone on "66 in 99" would do. The fuel to do this mileage cost $116.94 (approx. £75) at an average consumption of 29 miles per gallon. The cheapest standard unleaded fuel I found was 89 cents per gallon at Rolla, Missouri, while the most expensive was $1.20 per gallon at Malibu, California. The average cost of the fuel I used was $1.07 per gallon.
Meals for one person for 24 days, eating at a broad cross-section of restaurants from roadside cafés to high-quality establishments recommended in the guides cost $591.33 (approx. £370). This included buying breakfast every day, but breakfast is covered in the basic price of "66 in 99", so again you should have no trouble keeping below this figure.
I made over 400 photo stops along the way and would have made more but for the budgeting of both time and money. My only other unavoidable expenses were entrance fees to the few attractions that charge ($39.50 in total), and expressway and bridge tolls ($6.25 in total). You will no doubt also want to buy a few guide books and souvenirs along the way.