It very much depends on the countries you go to. Europe is EXPENSIVE. South America - not so much. North America - EXPENSIVE. Central Asia - not.
I spent 4 months in South America doing pretty much whatever looked fun, and eating well. I initially stayed in the cheapest hostels possible, but quickly realised there's a reason they're cheap - had some interesting ones, that's for sure! After that I'd pick the cheapest popular ones. However, if you do this, eat from the supermarket, take cheap buses, and all that, you can very easily do 20-25 Euros a Day. A bit harder in Chile, much easier in Bolivia.
This past summer I went from London to Mongolia overland. Western Europe hurt a little. The Baltics (Latvia, Lithuania) were great, then Estonia which uses the Euro pushed rates up a bit for food. Then I hit Finland, and goodness it was expensive - all the Nordic countries are.
Russia is more expensive than you'd think in the tourist cities - St Petersburg and Moscow. Once you're into Siberia, things are much better. And Central Asia - the 'stans, and Mongolia - accommodation is very good value, and so is food. Transport depends on your bargaining skills in Russian / local language ;) And eating from the markets can save you a lot!
Again on this trip, it varied. In Europe, you needed 20 Euros sometimes just for the hostel. In Uzbekistan, it was much better. My accommodation in Dushanbe, Tajikistan was one of the more expensive ones, but it meant I had wifi which I'd been missing for a couple of weeks.
Of the RTW travellers, I find a lot of them talk with glee about the cheap prices in India and SE Asia. Some are shocked by the prices in China being expensive in tourist spots - more so than Europe. And some are also surprised at how expensive Australia and NZ are (and so are the locals, I can tell you! ;)).
So it all depends on your style of travel. You could easily spend US$30,000 and have an absolute ball. You could also spend around 6 grand (500 a month) if you're really frugal and careful about what you do/eat, and where you go. You couldn't do that easily in Europe.
It's all part of the fun. If you come home early, it probably just means you spent it and had a great time. If you can eek out the cash, you'll have had more experiences, just not expensive ones ;)