Location/Budget/Interests
3 very important things for us to know before we can really be of help to you:
Where are you starting out from?
What is the approximate budget you're allowing for the trip?
What types of things are you looking for along the way?
I generally agree with AZBuck's daily figures (assuming that you'll be staying in motels.) I have always used a range of $60-$100 each per day for a 2 person trip with lodging in motels. If you're camping, you can slice quite a bit off the budget, as you'll not only save on lodging costs, but food costs as well (camp food is significantly cheaper than restaurant food, even if you go the very unhealthy route of fast food.) $40 each per day is very reasonable if you're camping.
Gas, comfort, and entertainment. Those are your three biggest costs. The first, unfortunately, is a constant. I'm in the Bay Area of California, and they're warning us about $4/gallon by the middle of summer. The other two are very flexible, depending on your preferences.
Good hiking,
LDTL