It's only slightly more expensive to buy the correct ticket for that zone, but it is good to be aware. You can still by a 'carnet' (pack of 10) to save money. By the way, when I was there most people didn't even use the turnstiles and instead walked out through the open gate.

The nearby Porte D'Orleans stop is inside the Paris zone, but it's not as convenient (unless you happen to already be near there). I'm not sure about the newer stop that tbm13 mentioned.