If you condition the battery when you get the phone (if they want to set it up in the store, tell them to plug the charger in before they turn the phone on and keep it plugged in until they turn the phone off) and keep the battery from dropping below 40%, you shouldn't have to replace it after 3 years. (I still have a couple of more than 10 year old lithium batteries that give me better than 95% capacity).
As far as the price, the battery itself shouldn't be more than $30, but with labor, you might be looking at twice that or more. (Which is one reason - the other one is that I don't like explosives in my pocket - I'd never buy a phone with a non-removable battery.)