Same issue with my 2008 Rogue (nearly 8 years old). It was getting so bad that several times I had to put the transponder near the door and more importantly by the ignition switch to start. So, I went to Battery Plus several months ago to get the battery replaced in the Rogue. While there I had them check the C-Max battery as many time I couldn't unlock the door unless I turned my right side towards the door (FOB in right pants pocket). The FOB had to be maybe within a foot or so to work consistently. Battery Plus checked it and suggested I replace it also which I did. No more issues with opening the C-Max door.
My C-Max was out of the B2B warranty (over 36k miles). So, I got just shy of 3 years life out of the C-Max FOB battery. My wife's C-Max FOB still seems okay at 37 months. IIRC, the cost was around $5 / $6 or so per battery at Battery Plus. I could have probably gone to Walmart and found a pack of 2 for the same price as one at Battery Plus but then I'd have to open both FOBs up.