Per capita emissions from food in UK (metric tons CO2 equivalent) 2.7
Annual reduction from a vegetarian diet (tons CO2 equivalent) 0.59 (ibid.)
Cost of a vegetarian-year $11 US
Cost of preventing a ton of CO2-equivalent emissions $18.52 US
Cost of a metric ton of carbon offsets
Carbon Fund: $11.00 US
Terrapass: $13.09 US
One of Brian's essay's includes a higher estimate of what a non-meat (vegan in this case) diet prevents in greenhouse-gas emissions (the link to the study that provided the figure is dead). A vegan diet doesn't prevent much more GHG emissions than a vegetarian one, so the discrepancy may be due to differences between the UK and US or the authors' methods. If we use the 1.5 tons/year figure, the cost of preventing a ton of CO2-equivalent emissions falls to $7.33.
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. So donating to the Humane League may be a more effective way of reducing emissions than buying carbon offsets. Of course, there could be some problems with the survey data that we don't know about.
I wonder whether there's any way for animal-welfare organizations to attract environmentalists' dollars without changing the message. Lobbying and activism are probably more effective at preventing GHG emissions than buying carbon offsets, so animal-welfare organizations couldn't compete with environmental organizations directly. But GHG reductions would at least be a factor in their favor.
[Edited to fix the second-to-last sentence. Thanks, Alan!]