Landing 300 rockets means SpaceX has preserved 2,700 Merlin rocket engines. In round numbers, the dry mass of a Falcon 9 first stage is about 50 metric tons, so the landing of all these rockets has prevented 15,000 metric tons of metal and other materials from being dumped into the oceans—the equivalent, in mass, of about 100 residential homes.
It’s amazing how routine the landings have become. With Starlink launches switching to Starship in the next couple years, and each Falcon 9 booster doing 20 flights (and likely more), I wonder how many more boosters will even need to be produced. They could probably do a few hundred more launches just with their current stock of boosters.