Subject: planning a budget

I'm a longtime lurker, but this is my first time posting to the list. My wife and I are in the preliminary stages of planning an extended backpacking trip next year. We're planning on going to India and Thailand--and perhaps other stops as well as time and money dictate. Neither of us has done anything like this before (which was why I signed up for the list in the first place), so any advice that you have beyond the scope of the question would also be appreciated.

I'm trying to plan a budget, and depending on where I look, I find wildly different estimates of how much day-to-day expenses will set me back. We are trying to travel cheap, so as to stay gone as long as possible. As of yet, we have no real itinerary, because we don't really know how much we will be spending in each place.

I'm a journalist, and she's a nurse, so we won't have unlimited funds (to say the least). Does anyone have any advice on how to plan a trip like this, particularly budgeting?