Not a real solid answer on the cost, gut reaction is 50 to 100 bucks. Maybe someone will read this shortly and have a good answer on the price, someone who's had to buy one. It does need the key, though, as the battery in the key is only for remote unlocking, has no effect on the transponder in the key. The transponder works like one of these chip things you can use to get gas, or like what they are using on toll roads to pay for the toll without stopping. It uses no power at all. Glad the diagnosis worked. Had that a few times now. If I think that's what it is, I have to tell my boss to call the owner and get the other key in here. the owner always reacts like "what, you LOST my key??!! Hard to convince them the key might be the cause. Brave new world, eh?