The author says the product is hosted on two VPS servers, a managed database, and a floating IP; I doubt that'll be a significant part of the costs.
If that bill includes (business) licenses for software and subscriptions, it's pretty reasonable. I'm sure it can be done cheaper and I'm sure the author has looked into this as well. Maybe it's not worth the effort, or maybe there's some other blocker preventing migration to cheaper alternatives.
Maybe $2500 of monthly profit is enough for the author not to bother messing with the code base? Maybe the fact the reported revenue has doubled [1] over two months has something to do with it?
[1]: https://casparwre.de/blog/python-to-code-a-saas/ vs https://casparwre.de/blog/webapp-python-deployment/
