There's a free tier limited to 5,000 events/month. The "Deploy" tier is $5,389.20 per project/year, plus event charges, plus a "connectivity assurance" charge.
Still, you could get a lot done with the free tier if you didn't overdo the traffic. Every half hour, "Soda machine #5621, no alarms, outside temp 82F, inside temp 36F, cash $75.25, stock level for Diet Pepsi 4, stock level for Sprite 50..."