Cost Estimating: Add flights, rail/bus passes, ground transportation, rooms, food, health insurance, gear, visas, vaccinations, entertainment, storage fees, shipping souvenirs and photos, and miscellany (entrance fees, snacks, bottled water, postcards, postage, bike rentals, tuk-tuk taxis, laundry, etc.). Do not forget to reserve money for activities, such as diving, bungee jumping, or skiing -- once you are on the road, living on a daily budget, you may find it difficult to spend that kind of money during the trip, if it isn't already in the budget. Keep in mind that airport taxes will generally be $10-$30 every time you leave a country by plane, and that there are sometimes departure taxes for domestic flights.
Realistic figures for these numbers depend on your preferences, and should account for inflation. Make sure you have enough reserved to cover returning expenses of a car, room, electricity, water, phone, food, and miscellany. Working your way around the world is covered later.
Available Funds: Add your assets, subtract your debts.
See the Cost Estimating Sheet if you want to run the numbers.
See Budget Your Trip if you want to get detailed about it.
There is no good "rule of thumb" for estimating the total cost of for a given travel time. It depends on your spending habits, the relative cost of each country, the season, and the amount of time you spend there. Because transportation is such a large factor, the cost will NOT be half as much if you travel for half the time. Don't forget to reserve a few thousand dollars to get your life back together when you return home.
All continents have both expensive and inexpensive areas, therefore broad generalizations may not be accurate, but they are a good start for estimating purposes early in the planning stages. Australia is a little more expensive than the US, and you know about Japan. Taiwan and Tahiti can also surprise you. Asia can be very cheap. Outside of the big cities in SE Asia, the "budget" RTW will average $10 per day. Europe averages $50 per day. My wife and I averaged $5000 per year each, which is a good estimate to work from if you are travelling in the more affordable countries, although it is possible to travel for less.
[These costs are just a starting point for discussion. I may delete them if enough samples, like the ones in the paragraphs below, are added to the Abstracts section at the very end of the guide.]
"I found India and Nepal to be cheaper than Southeast Asia. I spent about $8 per day in India, about $10 in Nepal, and about $15-$20 in Thailand and Malaysia in 1993." <Dave Patton>
"An interesting pattern I saw on our trip was that no matter where in the world we were, one night's lodging cost the same as 3 meals. For example, in Thailand we spent $15 on a bungalow and $15 on food (3 meals each). In Europe, we would spend $15 each for a bunk in a youth hostel ($30 lodging) and $30 for food (3 meals each)." <Russell Gilbert>
In Europe, the key to staying inexpensive is to stay south or east. The cheapest will probably be Czechoslovakia, Poland, and the former-USSR. Spain, Portugal, and Greece aren't as affordable as they once were. England, France, Italy, Germany, Switzerland, and Scandinavia can eat your budget up quickly. Paris, London, and Rome can shock your budget. North American metropolises can be as expensive as Europe.
"In Africa, I can say that Congo, Gabon, Rwanda, Burundi, Zimbabwe, and the Central African Republic are expensive compared to many other parts of Africa. Sudan as very cheap, as well as Zaire and Tanzania." <Mats Henricson>
How to Travel the World for Free is a good place to start, and if you find yourself in a shortfall, try How to Earn Money When You Are Broke on the Road.