Assuming there is a not a strike. We have 2-3 days of transportation strike coming up again this week. RER traffic will be cut in half if we're lucky. Also, factor in the 10-15min frequency of normal RER into your trip planning. I'll say it again, I would never come to Paris as a tourist and not stay within the Peripherique. Any cost savings is just not worth the extra commuting headache. If you have to nickel and dime it that much, you may want to save Paris for a time when you can increase your travel budget to fully enjoy the city.