Meta charges per message, and these prices then again depend on the message type and destination country.
You will only be charged for the first message, which will open a time window where you'll be able to reply to a user if they replied to your first message. All subsequent messages after the user replies are free, per Meta's terms.
There are 3 message types:
Utility: designed for sending notifications.
Authentication: for sending one-time passwords.
Marketing: for everything else.
Finally, the exact pricing will depend on the destination country, where some countries are more expensive than others.
We have created a tool to help you calculate how much you will spend based on these factors.
If you have an account, use the calculator app.
If you don't, you can use the public calculator here.