Do you have any friends or family members who are members of AAA? If so, get them to get you some highway maps for free/low cost. I used this method to go cross country, coupled with some additional detail maps for larger cities.

I have only used the Pacific Coast Adventure Cycling maps, but found them to be very helpful. They also were the only thing remaining after my tour - my other paper maps were either worn away, torn or crumbling from getting wet in my bags.