If you stick to the route, traffic is only an issue in a few places. Many people are seduced by a short-cut, however, and traffic can be bad on these short cuts. You will avoid traffic (and increase distance and hills) by sticking to the route. I encourage you to do so. Some people prefer light traffic even if it comes with no shoulders, and others prefer wider shoulders even if it comes with higher traffic. The ACA routes generally prefer the former (as do I).
I did the TransAm with zero flats and the Northern Tier with only one. This can be accomplished with good tires, not riding too close to the edge of the road, riding around road debris, and brushing your tires off every time you take it off pavement.
I did the TransAm on $16 a day, which I'm sure is below average. This is possible if you stay out of motels and expensive campgrounds, buy most of your food in grocery stores, and start with a bike in perfect condition. Luckily, the TA has a lot of free places to stay and they are identified on the ACA maps (which is a good reason by itself to use the maps). I agree with Pete, however, that you should budget about $30 a day (less if you are sharing expenses with somebody). I only count expenses from wheel-dip to wheel-dip, nothing that comes before or after.
Eat whatever you like, but eat often. My cross-country trip this year was powered mostly by sandwiches, fruit and cookies. On most days I would put two apples and two bananas in my handlebar bag and a large sub sandwich in my pannier. You should be aware, however, that fresh food can be hard to come by along the TA in Virginia and Kentucky, so you have to be willing to eat junk when only junk is available.