I purchased my 2016 Outlander with only one key fob. I'd like to get another one. I was just quoted about $500 for a key fob replacement at my Mitsubishi dealer.

Is there any reliable cheaper replacement?

 

Your title is a little misleading. It's not a cheaper "battery", but a cheaper key fob. You might consider changing to get more responses. I clicked in wondering how your car battery died so fast. Anyway, as far as I know, key fobs are only available at dealerships. This goes for any car that uses a key fob. Also, that is generally the price of these ($400-$600) regardless of the car. It is definitely overpriced, but I am not aware of a way around this. When buying a used car, that's how much you should take off of the purchase price if missing this expensive item. Sorry I couldn't give better news.