OK - so we just got back from our trip and I figured I'd write a follow up, since our trip seems like a pretty standard coast-coast trip that could be used as a prototype.
Traveled 7960 miles in 28 days, spending a week going from coast to coast, two weeks in cali, and one week getting back.
Spent $4700, which breaks down as such:
$1500 on lodging (but we spent roughly half of our nights at friends/family, the other half in $100 a night hotels/motels)
$1000 on food (family fed us occasionally, probably paid for 3/4 of our meals)
$900 on various expenses: sightseeing expenses, souvenirs etc.
$1300 on gas (at about 4.05 a gallon avg, roughly 25 mpg, expected to get only 20 mpg so we had lots of left over money here)
Major visits included : Grand Canyon, L.A., Monterey, San Francisco Bay Area, Yosemite, Denver, Chicago.
All in all the trip was a blast.
Hope this helps anyone planning a similar trip.