For summer where you will use the bag for several months, I suggest synthetic instead of down. During summer camping you will be in highly humid conditions in the tent every night and the insulation material will absorb a lot of moisture. Although a synthetic bag takes twice the amount of room packed as a down bag, a three season or summer bag is small enough that the extra volume should not be too critical.

Even a synthetic bag should be dried out, when you get to the campsite each night and pitch your tent, if the wind is not too high you can throw your bag over the tent and let the sun shine on it to dry it out a bit before the sun goes down. But once the sun is setting, get it into the tent before dew starts to form.

For a trip across the country, you should not use price as the determining factor because of the amount of use it will get.