Hello Sean 
A subscription is valid only for a single machine, it can be moved by one server to another but you can’t use it on both servers at the same time.
I also didn’t see where 85$ comes from, the Crostino subscription is 48€, about 54$.
My advise is: just install NethServer and use it. When you are satisfied, you can buy a subscription for your most important server or … just to support the project! 