You can refer to the article: how much a dyno cost?
In short, you need to know Heroku charge by dyno hours. And there are 2 type of dyno:
- Web Dyno
- Worker Dyno
And for each app, you are given 750 free dyno hours!
So if you run 1 web dyno for 400 hours, and 1 worker dyno for 400 hours, that means you spend 800 dyno hours. Subtracting from the free 750 hours, you would therefore be billed 50 hours.
Dynos cost $0.05 per hour, so 50 hours cost you $2.50.