A website left alone for two years still works. A mobile app left alone for two years often does not, and the reason is that you are a guest on two platforms that keep moving. That is where app maintenance cost comes from.
How much does it cost to maintain an app?
Platform changes. Apple and Google both raise the minimum requirements they will accept for new builds, and both eventually stop supporting old ones. At least one release a year exists purely to keep up, and it does nothing your users will notice.
App store fees. Modest, and worth remembering: a developer account per platform. The Apple account is renewed every year to keep the app available for download; the Google account is paid for once, when it is opened.
Servers and services. The backend the app talks to, whatever it uses for push notifications, file storage, and anything metered.
Certificates and keys. Signing certificates expire. When one does and nobody knows where it lives, a routine update becomes an emergency.
The bugs your users find. Not a defect of the build — the tail of any real product meeting real devices. How we handle them for apps we build is part of our mobile app support and maintenance services.
For a small product, the app maintenance cost per year is a few weeks of development plus the running costs. For anything with money moving through it, more, and it buys you the ability to respond when a payment provider changes something on their schedule rather than yours.
What happens when nobody pays it
The app does not break dramatically. It drifts. A new phone shows a layout bug. Notifications stop arriving on one manufacturer's devices. Then the store warns that the build targets an unsupported version, and the update that would fix it requires upgrading a framework two major versions, which requires touching everything.
At that point the choice is a small project or a rebuild, and the small project no longer exists — because two years of platform changes have accumulated into one large one.
Keeping app maintenance cost low
Ship a release every few months even when there is nothing to add. Keep the pipeline warm, the certificates current, the dependencies within a version or two of the latest. Each of those releases is boring and quick. Skipping them is what turns maintenance into a rebuild, and a rebuild is the most expensive way to own an app that was working fine. Those boring releases are exactly what a support retainer buys, and they cost a fraction of the rebuild they prevent.
If you have not read it yet: getting through app store review the first time.