To build on what other people are saying, the $100 also is supposed to cover the cost of running servers and networks that the app has to use for the more unique, advanced features. Those are all things that will cost the developer to keep up and running - it’s honestly nice that you even have the option to pay for lifetime since, once you’ve used the app for six years, the dev is going to start losing money off of your using the app.
To build on what other people are saying, the $100 also is supposed to cover the cost of running servers and networks that the app has to use for the more unique, advanced features. Those are all things that will cost the developer to keep up and running - it’s honestly nice that you even have the option to pay for lifetime since, once you’ve used the app for six years, the dev is going to start losing money off of your using the app.