There's really no way it costs them more than $3M, and many civil cases cost way less. They've already lost more than what I'd consider a reasonable upper bound. Besides, they're not a very small business, so they ought to have set aside money for legal events, and they might even have insurance to cover it.

(I realize that it's absurd and inherently unjust that the legal process is so expensive.)

IMO, even if it just gets the offending poster deleted, it would be money well-spent. The marketing/PR hit is just brutal. I blame Google for this.