For today's Social Saturday I want to present Volunteer Latin America. If you think about volunteering without spending a fortune to work, than you should look at this website.
"Want to volunteer abroad for free or at reasonable cost in Central or South America? Volunteer Latin America is an environmentally friendly organisation that helps independent volunteers find free and low-cost volunteer work abroad."
I think that it is definitely worth it to pay 24,50 Pounds for a "vounteer guide" of about 20 to 40 projects which are matching your specific requirements and where you don't have to pay a lot of money to help there :)