How much money would a cross country roadtrip cost?

I'm planning on taking a cross country road trip in 3 years (I'm 15 right now) starting in San Diego, going up to Seattle, down to AZ... Pretty much zig zagging across the country until I get to Massachusetts, then going straight to Flagstaff AZ. My question is, how much would this cost? I would be traveling in a ford explorer, staying in a cheap hotel maybe once every 3 days. I also won't be eating fast food all the way, I'll mostly pack the food I eat. For only the road trip (not including museums/art galleries/attractions I'll be visiting), how much money will I need?