Thanks so much for the question. Any Marketplace installation at DigitalOcean would require choosing a Droplet plan from https://www.digitalocean.com/pricing/.
There is no additional charge for this installation. The price is only for the chosen Droplet plan.
Here is how our billing works:
At the end of each calendar month, we calculate the cost of your server based on the size of your droplet. All droplets are billed at an hourly rate up to a maximum monthly cap, which we have set at 672 hours (28 days).
Therefore, if you use the server for less than the monthly cap, you will be billed for each hour that you used it. However, if you use your server for more than 672 hours during that month, you will be billed at the monthly cost, which will be no greater than the cost of 672 hours.
Additional services (automatic backups, etc.) may lead to further charges.
Hope this clarifies things!