Once you buy a Safe@ device for X # of hosts, the only thing you pay for on top of that is the subscription services - dynamic DNS, automatic firmware updates, anti-virus definitions, etc. IF you choose to use them, they will need to be licensed.
It's nice to have a site-2-site VPN and not have to use SecureClient when home so if you buy a used one for your home office, you should be fine. I bought one for myself and use it at home with no subscription services. My only initial concern was firmware updates, but since I have a real support contract and several edge devices, I can download new firmware and manually update code. As previously stated by others, the hardware and firmware are identical to the Edge boxes.
Go
here to see pricing for the Safe@ service subscriptions.