Machka has a good point. I did five days in my home state for $5 recently, with groceries from home. But my Northern Tier ride last summer took 55 days and $1500, nearly $30/day, about half in groceries and half in camping/lodging, not including travel to/from termini. Bike maintenance cost less than $25 (one chain and one spoke) after spending about $150 on the 20 year-old bike before leaving. I was pleased with how little it cost--I had budgeted over $1000/month (my average cost on long hiking trails). I'm not sure why it was so much cheaper than expected, about $750/month.
I had a warmshowers guest the other week who is spending about $5000 a year on a three-year trip so far, riding about 7000 miles per year, at a per mile cost of about 8/10s of a penny. That's amazing. He was healthy, happy, and had great gear.