I just found our trip log from last summer's trip. Perhaps it will be helpful to someone trying to plan a similar trip this summer!
1837 mi from our house in CA to mid-Missouri via I-15 and I-70
1891 mi from mid-Missouri to our home via I-70, I-35, I-40, I-17, I-10 and I-8
4647 mi total on the car (3729 round trip plus 917 sightseeing)
$380. fuel (premium unleaded in most places, occasionally mid-grade)
$109 sightseeing fuel
$377 5 nights motel (all the rest were with family)
$227 travel and layover day food
Gas was $3.03 - 3.25 gal for premium at that time
$75/night average for motel (okay, $49.10 was lowest, $102 was highest for 2 nights)
$32/day food budget (2 meals - breakfast and dinner most days
Gas = $3.10/gal at 22mpg (we averaged 26 mpg)
Motel - $75/night
Food = $60/Day
For 2011, we have had to up the gasoline budget. OUCH!