How good is doing “good”?

I'm conflicted. On the one hand I want to gain incredible experiences, new skills, lasting memories, and discover more about myself while I travel. On the other hand: I'm broke. This dilemma brings me to the idea of voluntourism.

I have become increasingly interested in doing volunteer work (particularly in wildlife rescue, marine research and nature conservation) in many of the countries I would like to visit. I've looked at a ton of companies that offer the opportunity to spend a month or so living, breathing and experiencing these amazing things whilst helping out, but they all cost an arm and a leg.

So now I'm left to wonder, how much good do these companies actually do, and how much is just making foreigners not feel so bad about destroying the rainforest during the other 11 months of the year. I'm also wondering if it's worth going through a company to get the experiences I desire, or if there is some way to find a local company doing the same thing, but without the extra costs (so that the money actually goes to the cause instead of marketing costs).

So now I'm stuck because I can't get the idea of doing something interesting, amazing and potentially helpful as I travel out of my head, but I also can't afford to even think about doing it, let alone actually shell out the $2000 for a couple of weeks in the Ecuadorian amazon.

I need help. Which company should I go with? Is there another way? Will it actually do any good, or will I just be adding to the deforestation of the world? If anyone knows of any alternatives, I'm all ears.