Honestly I think 20 usd is reasonable. I think people here are being weird. That's like the price of a book or indie game on steam - surely an app that you read on every day is worth that?
Also Lemmy will continue to grow so the value will just increase.
Are you looking for Ultra lifetime or ad-free lifetime? If you are looking for adsfree lifetime, it's a different option and obviously not Ultra lifetime. They are not the same.