The cost model also assumes non-criminal behavior. But for most cases where somebody would be motivated to find collisions for breaking a cryptographic scheme, they're unlikely to be restrained by the normal boundaries of the law.

IOW, for a criminal organization (or just a single criminal) with a huge botnet the direct cost might be closer to $0. Of course there's opportunity cost--botnets are often rented like a cloud service--but that's the case for everything.