Cost of Worm Attack Protection

I was hoping to get some estimates from folks on the costs of defending networks from various worm attacks. It is a pretty wide open question, but if anyone has some rough estimates of what it costs per edge, manpower vs. equipment costs, or any combination thereof it would be of great assistance. We are doing some simulations of attack and defense strategies and looking for some good metrics to plug into a cost benefit model. We'd be happy to share the results if anyone is interested as well. Thanks in advance, sean