How much it cost me:
$1000/month (with free flight, not including Galapagos trip ($1000) and pre-trip spending which generally can be amortized over more than just the length of the trip because you'll use things like cameras and backpacks for a long time. For me, it should count as trip-only spending because I lose everything by the end of 6 months.)
How much it will cost you:
Airfare - This trip was the best reason I could think of to be a frequent flier. For just 35,000 miles, I was able to change my flight times and destinations, 5 or 6 times, and flew for free. Most U.S. citizens I met, flew for about $800. Europe is more. Australians should get round-the-world passes for about $3000 or so.
Pre-trip Shopping - Camera, Clothing, Sleeping Bag, etc.
You can easily spend $1500 before you get on the plane. Buy what you can't get there and key items that you don't want to spend days tracking down while you're on vacation. I spent about $1000 before I left but didn't buy any camping gear.
Room, Board, Etc.
Before you leave, you should determine your philosophy toward backpacker-style travel. One question to resolve is this: Is quality or quantity more important to you? You probably could make it on less than $300/month if you stayed in some little village in Bolivia and didn't do anything. If you spend your nights in a tent, eat cheaply, see the sites that don't cost much to see, you could keep it below $500.
I decided that I was going to be a value-seeking traveler. I didn't stay in the cheapest places nor did I stay at the Ritz of Rio. I talked to people, read my Lonely Planet guide and looked for places that were clean and comfortable, sometimes even with hot showers. For some comforts, I gladly paid a premium. El Cheapo might stretch the budget farther than me, but he didn't enjoy himself to the same extent. Cheaper travelers would hear about how I spent extra money here and there and say that I could have had an extra 3 days in Bolivia, instead of the nice dinner in Santiago. You know what? I would have gone home after 6 months either way. It still makes me laugh sometimes when I think about those poor, starving travelers, rejoicing at how they spent their 3 extra days in Bolivia.
A few splurges of mine:
I paid $67 or whatever it was for a pack of sherpas to guide me through the Inca Trail, carry my backpack up the steepest incline, cook my food, and set up the tents. I saw others struggling more while I made it easier for myself for just a bit more money. That's value to me.
I blew about $40 one night to go to the best club in Buenos Aires, called Buenos Aires News. Although it was not much different from many places that I've been to in the U.S., I now know what it's like. I lived it up. I have no regrets. Those extra days in Patagonia would have been too cold anyway.
Brett and I spent $139 to fly from Buenos Aires to Patagonia. It took about 2-3 hours. I don't remember well because it was just another flight. We met some friends there who had been on unpleasant bus rides for 40 hours through uninteresting, never-ending lands. They saved $39. They looked tired when we first saw them. We were fine, and we had and extra day to see a couple of small Patagonian towns. I guess they didn't save much there. (As a side note, I wouldn't recommend flying everywhere. Part of the trip is about seeing the land. Riding the bus from place to place is the way to do this best. Sometimes, though, I had seen enough of the land or knew that driving through Iowa and Nebraska made up for not going by land through some desolate regions.) Just about every flight has its pros and cons. That flight was a no-brainer.
I went to the Galapagos Islands, even though I knew that, compared to what I had spent to that date, it wouldn't be a good value. I was right. However, sometimes you need to do something just to not have to go back to do it again. That's a big reason for why I went to the Galapagos. Any of you ladies who are excited to meet me may be disappointed to find that I'll never take you to the Galapagos. Never again!