Where are you flying from? Sometimes you can get some great deals (like visiting Asia from the west coast, various places if you're flying out of JFK in NYC)? Is there a time of year you're targeting?
How much time do you have & what's the budget (I wouldn't go somewhere more than 10 hours away for just a week, but if you have 3 weeks, it adds alot of places in).
If you have the money for flights, some cheaper destinations are:
Asia: Nepal, Thailand, Cambodia, Malaysia, Indonesia, Philippines, Vietnam
Europe: Eastern Europe, Georgia, Armenia, Turkey
Latin America: Central America, Colombia, Ecuador, Peru
These places are easily to be under $100 a day and probably under $50 a day for some of the places depending on where you stay, eat, do.