How to get really cheap an upgrade to business class? For example with air serbia?

I would like to fly for one time at least, with business class, but budget is 50 euro. I heard from others that i was possible sometimes… but how to get this? Not with every offer? Moet offers are starting from 110 euros… is there another method to this this?

Thanks in advance for youe advice guys! Mich appreciation 🙏🏼🩷