To my knowledge, the Libre can’t be extended. People who have tried seem to only get a day or two of extra use. I think in the case of the Libre, it’s a battery issue.
Even though the Dexcom is cheaper than the Libre when stretching supplies, the Libre is far cheaper than Dexcom when supplies are used as directed. Like, about a third of the cost, which is huge when you’re paying for this stuff out of pocket. The sensors alone are half the price ($85 for a Dexcom sensor that lasts seven days versus $89 for a Libre sensor that lasts 14 days), and they increase even more when you consider that there are no transmitters to buy and the reader/receiver is a fraction of the price ($49 compared to $499 that I was quoted for a G5 receiver).
If the Libre remains this cheap when it becomes a full CGM in its next version, I think Dexcom is going to have some serious competition.