a friend working at a nonprofit here in BSAS told me that the idea of volunteering as we think of it in the US or UK is a bit foreign to argentines, so the reason these companies charge is to make sure that you have an experience similar to what you might experience in the US or UK (i.e., substantive work.)
helpargentina was recommended to me by this friend if you haven't made a decision on using one of these agencies.