Until someone can give a better official answer, this is what I see, and has been my experience:

https://www.digitalocean.com/pricing/
“Do I have to pay the cost of the server every time I create a new one?

No — you only need to pay for 1 hour of usage. Spinning up a new 512MB server just to test something for a couple of minutes will only cost you $0.0074”

I have two relatively idle VPS, and the charge for each is less than the contracted monthly fee limit.

Thank you for the answer, I already cjecked the biling statements and is charged hourly.