TPMS batteries usually only last 7-8 years according to my research, so you got a good run out of the '06. Unfortunately, I don't believe you can service the batteries, so it's a replace. I recommend doing all four at once.
I've got an '08 Pathfinder with 120k miles and one sensor is intermittently giving a low battery signal. The local NTB quoted me $60per, parts and labor, or about $260 out the door with tax. You can get them for $30-$40 online, but you still need someone to break the tire down for the install.