That math assumes that the only two options on the table are either accept funding from YC or don't accept funding at all. The benefit you get from multiple investors is not orthogonal because you get diminishing returns from extra funding and because of the potential (for better or for worse) for interplay between investors. So, if you have a different investor offering you different terms (or even the potential for such), as is more likely to be the case for a more established startup, then things get more complicated than that simple cost-benefit analysis.