Cheapest way to host backend for small-scale app

I'm building an app that will never have more than 80 concurrent users, so scalability is not a concern. The app will have some network features, so it needs a backend on the internet. The main priority for development is low upkeep costs. For example, if there is a way host the backend service with a one-time $1000 payment, that is preferable to having to pay $10 per month to host this app. What's the best way to do this? Context: Junior Ungerdrad in CS