One reason the AR-15 design is so popular is that it’s tough and reliable. When they’re properly maintained, they can last for several decades without the need for serious repair. Of course, you’ll probably have to replace a spring or a pin here or there. But for the most part, mechanical failure is not a major concern.
The biggest wear and tear expense you’re liable to run into is if you have to replace the barrel. But AR-15 barrels are designed to fire thousands of rounds before they suffer any significant wear. And even then, because the AR is a modular platform, the barrel is relatively easy to replace.