"We carbon offset our car each year for $99. In the past we were doing this at Carbon Fund however, I started thinking that for the same amount of money, the offset would probably be bigger by donating to Vegan Outreach as it has bigger and longer lasting effects not only on the environment, but for the animals. Also, you use your money so much more efficiently that we wanted to support all your efforts. So, here you go."
As we've pointed out many times (for example, "Beyond 'We Want' to a Real Earth Day Ethic"), a rational concern for "the environment" is based on the consequences of environmental change -- not the change itself.
However, looking just at carbon, Ben West has done this calculation of Vegan Outreach's efforts, concluding the cost per one ton of carbon removed to be approximately $7.40, lower than the offsets by Carbonfund ($10/ton) and Terrapass ($13/ton).
Of course, this calculation ignores the direct and intense suffering (and other environmental destruction) offset by everyone who follows a more ethical diet because of Vegan Outreach's work. And, as this analysis notes, "Vegetarians on average create more vegetarians, while carbon offsets don't create more carbon offsets. And I think the estimates also ignore people who reduce meat consumption."
Thanks to Brian for the link.