There is no licence cost and I have seen Marketo not charging the second instance while the first one is still in use.
But that might be a huge project, as you will have to migrate all the templates, programs and data. You will loose all activity logs...
Marketo wants to charge $25k as a cloning fee. I know that we will lose activity logs. Our integration is poor at best and I have no problem rebuilding everything over. I just don't like paying $25k for something that should be a standard functionality