The Prisoner's Dilemma. Classic, classic example where cooperation has a non-minimal cost - ie, the risk that they will defect against you, multiplied by the probability that they will defect, is the cost of cooperating.

VERY minimal cost (if any) of cooperating with a cooperating entity

And in the Prisoner's Dilemma, if you somehow specify the other entity is cooperating, then cooperating with a cooperating entity carries a cost still: the difference between "both cooperate" and "you defect against their cooperate" is the cost of cooperating there.

throwing cryptic opinionated darts

If your argument rested on some mathematical concepts, and one of them was an equation that you derived incorrectly, and wedrifid pointed that out, would he still be throwing darts? He wasn't telling you that you were wrong because he hates you, or because he enjoys ruining peoples' time on this blog, or any other sadistic personality trait, he was pointing out the flaw because it was flawed.

Cooperation is a non-zero-sum game that continues adding utility the longer it goes on. Do you deny that this is the case?

That is sometimes referred to as "mutually beneficial" cooperation:

Co-operation or co-operative behaviours are terms used to describe behaviours by organisms which are beneficial to other organisms, and are selected for on that basis. Under this definition, altruism is a form of co-operation in which there is no direct benefit to the actor (the organism carrying out the behaviour). Co-operative behaviour in which there is a direct benefit to the actor as well as the recipient can be termed "mutually beneficial".

• http://en.wikipedia.org/wiki/Co-operation_(evolution)