It's very difficult to estimate what the total tab will be as everyone's travel style and comfort levels vary greatly. Depending on the country and exchange rate, our daily budget averaged anywhere from $15 USD per day to $100 USD per day. When we totaled up everything in the end (including $700/month student loan payments while we were gone), we ended up spending approximately $65,000 for two years of travel. Considering what our living expenses were before leaving the US, we actually spent less money traveling than we did living "the daily grind" back home...of course, that's not being nearly as frugal as we have been during our travels.
OTHER TRAVELER'S BUDGETS
When looking through financial breakdowns from other travelers, it's important to consider that everyone travels differently. While one couple may average $100/day, they're eating in nice restaurants and staying in hotels with air conditioning and the comforts of home. Another traveler on a motorcycle might be averaging $20/day, camping every night and making there own food. Not to mention the difference in fuel costs on a bike vs. driving an Earthroamer RV.
Just ask Nicholas Rapp from the TransWorld Expedition. He found his Cost of driving around the world added up to $1 per mile, which totaled up to near $50,000 to see the world. Dan Grec, of The Road Chose Me, spent only $27,000 during his 40,000 miles in a Jeep. He mostly cooked his own food and stayed with locals he met through Couch Surfing. 95 Degrees South, spent $55,000 driving 28,000 miles in their Toyota Tacoma. These are just a few examples, but we have found that you can spend anywhere from $20k-$100k to go on an adventure like this.
YOUR MILEAGE MAY VARY
Another important factor is the exact route you end up taking. If someone wanted to take the fastest route, point A to B, you could drive from Ushuaia to Alaska in two weeks and less than 14,000 miles like the TDI Panamerica Expedition did. Alternatively you could spend 3 years driving in circles, taking your time until you reach South America, driving over 30,000 miles. Personally, we like the 2nd option better.