In Southeast Asia we are averaging a bit less than $25-30 for two people, no camping, few meals at restaurants, few buses/trains hops.
The big expenses are:
AIRFARE (this can be largely avioded as there are overland routes through Europe-Australia that rely on some ferries and perhaps creative thinking to get to Oz from Indonesia) but there is the hop to the Americas that can run @ $500 from Bangkok to NY one way.
HOTELS: They are downright cheap in Southeast Asia, Africa and Latin America (less expensive than camping in Europe and North America). Free/Wild camping is an option in most places and will save considerably.
www.warmshowers.org is a good option to save.
FOOD: Covered extensively in other recent threads. Short answer... it depends.
VISAS: Depending on nationality, this can get expensive. The visa for Vietnam cost $50US and an extension was another $25.
EQUIPMENT: I am biased. Buy
second hand and save.
GUIDEBOOKS: These are not particularly cheap and you cannot carry them for every country you plan to visit - so you must pay the local price - if you can find it. For instance, we foolishly bought a Thailand guidebook in South Africa for nearly double the cover price.
TOURS/TRIPS/EXCURSIONS: Often there are tourist attractions that are just beyond the reach of a cyclist or the attractions is better accessed by a private tour. These cost. For instance, a few days ago we took a local tour to the My Son ruins here in Vietnam. The trip cost $4US per person including lunch. Admission was another few dollars. We could have cycled the 90 kilometer round trip but decided to rest and take the tour.
MISC. Here are some of the things we've spent on recently: Laundry, tea, power spraying mud from the bikes, welding a broken braze-on, internet access, books, haircut, and deoderant... Remember, life goes on while on the road. Often those things that are cheap at home (oatmeal comes to mind because I miss it and would pay $10 a pound right now) will cost dearly in another country.