It really depends on the Droplet’s size and the monthly fee.
Additionally, if you don’t destroy/remove the droplet, you will still be billed for it even if it’s just offline. When you power off your droplet it is simply powered off. Your disk space and data is still there and is preserved.
Keep in mind that you are still billed for droplets that are powered off becase your diskspace, CPU, RAM, and IP address are all reserved while it is powered off. If you want to save a Droplet for future use take a snapshot of it and then destroy it. You’ll be able to create a new Droplet from the snapshot image anytime to bring it back online.
I’ll recommend checking out the following documentation about Billing: