Queller's 1985 version of Hamilton's rule
It is just worded a little wierdly in my opinion. The key line in the paper is: 'Fitness components are also defined for all individuals, for example, $C$ is defined, even for a non-altruist, as the cost it would incur if were altruistic.'

Essentially, if it doesn't matter what individual you are you always pay the same cost, then $C$ is a constant. This is pretty much the definition of a constant! The expectation of a constant is just the constant, $E(C) = C$. 

The reason it works is that to find whether or not the cost is realised, Queller simply multiplies by the focal individual's phenotype value $P$, which is either 0 or 1. So either the individual pays the cost or it doesn't.