Quite expensive, actually. When PC was starting and Kwesi and Steve didn't want any advertisements on the forum, I believe it was about $260 a month. Now they've got ads, as well as donations, which helps.
But, more on the subject to what Legacy said, if PC were to close for that reason, it wouldn't make much sense since PC has been up for quite a while, and for Kwesi and Steve to have gotten this thought now would be sort of odd. It's been costing them money for a long time now, since they've kept PC up and running, if one of them were to go bankrupt for one reason or another, I'd understand.
I doubt that the reason for shutting down PC by now would be financial.