So I dug around a bit and found this https://docs.particle.io/support/pricing/general-questions/#do-i-have-to-pay-anything-to-use-particle-mesh-devices-
Which has this quote:
All Particle customers will get 10 free gateway upgrades for prototyping and evaluating Particle Mesh. All local communications between mesh endpoints are free.
With 10 free gateways, you could build:
One giant network of 100+ devices
Ten different networks of 10+ devices
Five different networks with redundant gateways (Ethernet + Cellular backup, for example)
Which looks a bit more positive. Does anyone know how to link gateways together to make the giant network, in my case of 30 devices?