Since all of my work output goes to effective altruism, I can't afford any optimization of my meals that isn't about health x productivity.

Allegedly, vegetarian diets are supposed to be healthier, but I don't know if that's true. I also don't know how much of a productivity drain, if any, a vegetarian diet would be. I've personally noticed no difference.

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Assuming I go on eating one meat meal per day, how much vegetarian advocacy would I have to buy in order to offset all of my annual meat consumption? If it's on the order of $20, I'd pay $30 just to be able to say I'm 50% more ethical than an actual vegetarian.

It depends on what the cost-effectiveness ends up looking like, but $30 sounds fine to me. Additionally or alternatively, you could eat larger animals instead of smaller animals (i.e. more beef and less chicken) so as to do less harm with each meal.