It shouldn't cost them much.
CoH's website certainly is hosted in the same place their other websites are hosted, so it being there or not doesn't cost anything.
The only thing they'd have to actually pay for is the domain (cityofheroes.com), and even that is dirt cheap (and that's for us normal folk, for a company it's a meaningless price). And that's something they'd want to maintain either way - imagine the pancakestorm if someone else bought cityofheroes.com after it expired and used it to scam people... That would hurt their image, whether they were responsible or not.