Well, Your calculation is not appropriate.
Even when you don’t make money, the host still has to keep their infrastructure powered, up and running to keep your site alive. In addition to that, they too have to pay their staff to manage both the hardware and software.
About self hosting, a vps on digitalocean able to host discourse is approx $15/month for a small site. installers will install it for you for a fee of ~$100-150 and you can request the stable branch which is not updated as often and only needs occasional maintenance which shouldn’t take more than 30 minutes of your time. Even if you hire someone twice a year at $150/hr to maintain your discourse server, it still rounds up to approx $650 a year. If you do it yourself, hosting bill is only $200.