100% Free volunteer programs abroad ?

I'm wondering, after searching all over the net, if there are any truly free volunteering programs that pay for your plane tickets at least. I am not that wealthy in fact, I don't have much money at all, but I'm aching to go to Mexico or somewhere down there or Sud America and teach people english, or help build houses, or anything of that sort. I can probably raise money to pay for my stay but everything else, and plane ticket prices, I am looking for a program that will fund those.. Are there any? Every program I've seen is like $2500+.