We could also say that the relevance claim is a claim of how much the probability of the original statement would be affected if the argument in question were true.

Usually there is more than one statement in play, or more than one viable interpretation of a statement, as you point out with "Martians are green". So I prefer to think along the lines of "how much of the actionable value of the original statement(s) is affected by the new argument." This has the advantage of highlighting which elements of a cluster of statements are important, and which less so. It also highlights the possibility that participants in the conversation may see the relative importance differently.