I doubt if the $100 retail price of the unit (which means <$50 to the maker) covers the manufacturing costs. Spot are using the monthly fee to subsidize the cost of the unit in the same way as a cell phone.
I think Spot's business model was that a $100-150 initial purchase price and a $100/year subscription would get more market penetration than a traditional $800-1000 PLB and they would make the real money on add-on such as the $50 tracking service. Presumably as the technology advanced they would add more 'value-add' services like text messages etc.

There is a free public funded service, COSPAR, used by PLBs and ELTs and paid for by taxes from a bunch of countries.
This is more like the monthly monitoring fee for your home alarm system - it's not paying for the fire dept or police but it is paying for the operator to handle the alert and forward the call. (Or the ON-STAR system in cars)