This is a thing to consider. I found a good post about fellow travelers and how much their trips cost. Their per-person rates go from about $50 to $75 per day. Sounds like a lot, but it's only $18-27k for a year. ... which is less than I spend here in the States.
I think I can do it for the lower end of that range. I plan to travel mostly overland, and Asia (besides Japan) tends to be cheaper than Europe, Africa, Australia, and North America. (not sure about South America, I think it's comparable.) I've budgeted $20k. Course, I'll keep you updated once I actually start spending money.