Peyton Manning is 35, has had two recent neck surgeries, and the new contract he signed has out clauses related to the neck injury.
Cam Newton signed a 4 year $22M contract, $5.5M per season isn't that much to retain a future starting QB.
If they get Luck it's possible that they could dump Manning due to the neck and move on, but even if they let Manning play it out it's not like Luck would be making so much that they couldn't afford it. Hell, Tyler Thigpen is making $4M this season as a bad backup with almost no upside. They could then resign Luck or have the franchise tag to avoid losing him if that becomes a reality.