Short answer: right around $15,500 for 2013 and 2014, the two full years I've been on the road thus far. This is pretty close to what I paid living in an inexpensive apartment with a roommate before hitting the road - but this way of living is a lot more fun.
I mostly work-camp to pay my way so I stay in one place for months at a time, then do big re-positioning trips to get to my next gig. I work at national parks in the late spring-summer, Amazon in the fall-early winter, and then take Jan-March "off" (I volunteer in exchange for a free site).
I never boondock, will dry camp when getting from point A to point B, and get a steeply discounted or free site at my work-camping gigs.
My truck gets about 15 mpg highway towing, when towing my trailer I travel between 3,500 and 5,500 miles in a year - but when you add just truck driving (commuting, sightseeing, grocery runs, etc.) the mileage I put on my truck in a year jumps to about 16,000 total.
Health insurance, phone, mail forwarding, and vehicle insurance are my reoccurring bills (I'm debt free and rely on free public WiFi) and total about $267 a month.
I hope this helps! I also write a lot about topics like this at my blog if anyone's looking for more info.
Safe travels and happy trails.