If Mahomes contract can be used as a comp for anyone, it's Lamar. That said, until Lamar wins a playoff game, he's looking at whatever the Watson contract would be. He's not eligible until next offseason, and there will inevitably be a few more QB contracts handed out by then.
If I'm the Ravens, I'm locking down Lamar with a long-term deal as soon as possible. As we know, the cost only increases and even bad QB's raise the bar (looking at you Flacco), so the sooner we do it, the better the deal will be long-term. I'd rather sign him to a 10/450 now than have to sign him for 5/250 in two years.