If the utility of two people being vegetarian for a year is roughly twice that of one person being vegetarian for a year, and so on, then if they're contributing money to make more vegetarians, then they'll probably want to give a lot more than the cost of one vegetarian. In practice, the marginal value of a vegetarian year changes very slowly as the number of vegetarians increases, so if a consequentialist altruist is buying vegetarians at all, they're probably buying them in bulk.
Someone who did this might also become vegetarian or vegan themselves because it was useful for personally convincing others to do the same, or in order to feel more consistent. It would also be worth it for them to change their diet in addition to contributing money if their productivity, etc. didn't decrease by more than the cost of making another person vegetarian.
Their own quality of life would not be a significant factor in itself (though its effects on how much they accomplish would be), unless they value their own wellbeing more highly than that of others, or are more adversely affected by the diet than the typical person is (for example, it's more difficult to be vegetarian if one also needs to eat a low-starch diet for health reasons). If they're paying to encourage others to become vegetarians, and also believe that being vegetarian decreases the typical person's quality of life, then they must value the reduction in animal suffering more than the benefits the other person would get from eating animal products.
I notice that a large fraction of Effective Altruism people are vegetarian. This makes sense: in general Effective Altruists take moral issues seriously, even when that means changing your lifestyle. I'm not sure it's a good balance, though.
One way to think about this is to convert it into money. How much would I need to be paid to give up eating meat? All animal products? How much money would I need to spend on myself to be about as happy as I would be with less money but continuing to eat animals? I'd probably be willing to go vegetarian for about $500/year, vegan for maybe $2000/year.
It turns out you can probably pay to convince other people to be vegetarian much more cheaply than that. I estimate the cost of a vegetarian-year at $4.29 to $536 while Brian Tomasik estimates $11 with better methodology (which I looked at). This is by placing ads on facebook for a site where people can watch an animial cruelty video and ideally become vegetarian or vegan.
If you would get more than $11/year worth of enjoyment out of continuing to eat meat, why not give $11/year to convince someone else to not eat meat for you? Or give $50/year and be on the safe side?
(While you're giving money, you should probably give it to the organization that you think will do the most good with it, which I think is probably one of GiveWell's top charities. The nice thing about money as opposed to actions is that it's easy to redirect.)
I also posted this on my blog.