You pay for the nodes you reserve. The price is set by the spec class, the number of nodes, and the regions you choose, plus a one-month prepayment when you create the engine.
What sets the price
Three things drive the price: the spec class (the size of each node), how many nodes you reserve, and where they run. Cloud-hosted nodes with longer reservations cost less per month.
Prepayment, operating budget, and emergency balance
At creation you prepay one month. From there, an operating budget funds the engine month to month, and an emergency balance is held in reserve to keep the engine running if a payment is late. Both terms are defined in the glossary.
Where to see live prices
Prices can change, so check the pricing page in the console for current numbers rather than relying on a figure quoted here.
Related: How do I create an engine? Where do I see my charges and invoices? Glossary.