Cloudflare serverless infrastructure is a usage based model. It’s technically completely free to use, but the free plan is somewhat limited, which is why I recommend the Workers paid plan of $5/month. This gives you a decent included usage amount, but then they will bill for overages, which is reasonably cheap compared to legacy hosting.

It all depends on how much traffic the relay receives. If it’s a personal relay for yourself, family, or a few friends, you might be able to just keep it free or at the $5/month mark. If you’re wanting to scale the relay to have lots of users, then you might pay a few dollars more. I recommend looking up the pricing calculators for Workers, Durable Objects, and D1 database.