It’s called a loop out, and it depends.

First of all, you can’t just pay an on chain address from a regular lightning client.

So what you do, is send sats to a Channel partner that is connected to someone who will Provide such a service. With that, you need to pay the fee that the channel partner sets, the nodes between for liquidity, and then the provider for the service.

Some lightning clients obfuscate this service by just doing all of that on the backend, or are the loop out service provider themselves. An example is wallet of satoshi. That’s where they make fees for profit.

There are other ways to do it to, for example, you can pay a strike lightning invoice, and then use the cash they deliver to pay an onchain invoice, where you’d pay the miner fees and whatever minimal liquidity routing fees needed to pay the lightning invoice to strike.