Two to three years is typical of a battery, when placed under continuous use, whether it is in stand by or being used. They are not lifetime products, and considered disposable goods.
When I had a emt through Comcast, I removed the battery after we got rid of the phone service, due to we never lose power, and if we do, it is less than an hour, but can be longer in rare occurrences, in the case that we had a ice storm in the past six years, and a tornado around the same time.
Some people have forgone the built in battery, and started to use UPS systems instead, so that they can get longer time out of them. You really may not be able to find any pricing lower than you could locally with the battery shop in your town, but sometimes they are higher than you can purchase the unit online.