The cheapest major tourist destinations worldwide would be Egypt and Indonesia.
In Egypt, if you don't mind eating local food and sitting down with Egyptians instead of tourists (and who would mind? The food is excellent and Egyptians are effusively friendly), you don't need to spend more than US$0.50 for a meal.
In Indonesian islands like Bali, you don't have to spend much more than that for food, and you can relax in beautiful family-run beachfront hotels, in a private cottage all your own, for US$10 or less per night if you stay away from the most crowded beaches.
Of course both of these destinations can be as expensive as you make them, if you choose to stay at big-name international hotels and eat (boring) big-name international food. But for the value-conscious traveler who wants luxury on a budget, you can't do better.