I rode a 23 day roundtrip ride this year from western NH to St. Louis. I spent just over $500. I only spent 4 nights in hotels and ate out most every night/breakfast after the first 6 days of the trip was over. Granted I wasn't eating high priced meals. The spots I camped out at included trailhead parking lots, behind churches, behind truck stops, stone quarries, business parking lots, baseball parks, cemetaries, city owned parks(probably not legal but no one knew I was there, got there after dark), and...yes...behind big box stores(Wal-Mart and Lowes). Your best tool for finding campsite before you ever leave home is Google Earth.

Be willing to camp out and cook, it isn't as bad as you think. I wish I would have had a camp stove and sleeping bag with me as I could have saved money and the whole cost of the trip probably would have come in under $250 for a 2800 miles trip.