Even better, just use your old derailleur. It will work fine with newer shifters. Just a reminder, you'll also want to get cable stops that mount where your shifters used to be, plus new cables, a couple of ferrules, housing, bar tape, etc.

Not sure why a chain is more expensive than a RD. Maybe chains require tighter tolerances, better materials, or more manufacturing steps? As I've stood up to power across an intersection I often marvel that the thin little chain can take the multiplied torque I produce. And I'm really not that powerful -can't imagine what a pro sprinter puts out.