Scaling a monolith is almost always cheaper than migrating to microservices and scaling that.

When you split it into microservices you are adding a bunch of infrastructure that did not need to exist. Your app performance will likely go way down since things that used to be function calls are now slow network API calls. Add to that container orchestration and all sorts of CNCF-approved things and the whole thing balloons. If you deploy this thing in a cloud, just networking costs alone will eat far more than your $20(if not, you'll likely still need more hardware anyway).

Sure, once you add all that overhead you now may have a service that can be independently scaled.

There's also nothing preventing you from splitting off just that service from the monolith, if that call is really that hot.