I think the return on Burnes would be surprisingly high. I can't think of any precedent of a cy young being traded with 1.5 years of team control, at least not recently. There is of course the Greinke trade, and in retrospect I'm not sure the Royals got enough in that trade. What I've noticed in recent years is teams will truly pay a premium for an elite talent. It is however similar with elite prospects, you don't really see those guys move unless it's for like Mookie Betts or Juan Soto. Burnes does fall in comparable territory for me. A big club could trade for him in order to quickly give him the big extension before he hits FA and keep him long term.
Last year, Luis Castillo was traded with 1.5 years of team control. He's a very solid starting pitcher, but inarguably not as valuable as Burnes would be at the deadline assuming he performs to his standards and doesn't get hurt. The Reds for Castillo got currently the number 29 prospect in baseball, the number 44 prospect in baseball, and 2 mildly interesting throw-in types. That is the absolute floor of what we could get for Burnes. I think we would get one absolute elite prospect added to what was received for Burnes. So a prospect in the 5-15 overall range, a couple more between the 30-75 range, and a couple interesting players...probably more interesting than what the Reds received as those guys I don't think have much hope. That can of course get shifted around, if for example the Orioles went on a run and were willing to trade Gunnar Henderson...we might not get much else. I've seen a couple people note Painter/Rodriguez as the headliner, that makes a lot of sense based on the rankings.