cost/benefit isnt very useful if it's based on something subjective
Man, you're missing what we're saying.
Every decision you make in your life is based on weighing costs and benefits. It doesn't matter if you include things that are subjective in your decisions. It's important to point out opinionated statements in arguments that try to make general conclusions, but that's not what's happening here. iago is going through the costs and benefits that he's seen in the vegan lifestyle. He's not trying to convince anyone that his arguments are generally true or that his conclusions should be accepted by everyone.
Aside from that, an argument can still include premises lightly subjective, make a general conclusion and be legitimately be called a cogent argument. Classifying something as subjective doesn't automatically mean that it isn't acceptable to most people. For example, most people would agree that murder is bad. This is obviously a lightly subjective claim, but that doesn't mean it can't be used in an argument.
I also completely agree with what Ender said. You seem to consistently dodge what we're saying.