Steve,
if you have a virtualization environment (like VMware, HyperV...) in your locations you can run your gateway virtualized as virtual machine. If you are satisfied with the availability of your virtualization environment you need only one gateway license for a location. Maybee this is something better from the license costs.
It's hard to say what the best solution without knowing more.
Maybee if you can centralize all you hardware at one location you can use VSX, Running a bigger central appliance-cluster with all your firewall gateways as virtual instances.
Wolfgang