Mine is also low and every once and awhile the low gauge on the dash shows a lower amount of battery between the F and E display of the battery on the dash screen.
They are not cheap at the dealership, but its a simple 2032 watch battery. It cost me just under $3.00 for two Duracell's at Cvs. So its not a big deal to change. You just have to take the door key out of the fob and then separate the fob and then switch them out. Very easy.
I have not changed mine yet, just got the battery, but took it apart to find out what it was like inside and to find out the battery type.
If your battery ever does die in your fob, I guess everyone knows you can insert it in the fob holder under the dash to start the car only, it won't charge the fob battery though. I think everyone knows that though.
Just thought I would insert my experience and what I found out about the battery.