I live nextdoor to an immobilier here in Paris, and the prices always leave my jaw ajar. I wish I could offer some good advice, but there's so much outside of Paris. I was in Neuilly and Boulogne last week, which are very close to Paris, but judging by the houses I saw, I would say it's potentially more expensive to live there than in Paris itself! There are of course suburbs you really ought to avoid.
The RER needs to be your most important consideration in many respects, as it's going to up your travelling costs, and you have to offset this against the reduction of living in a suburb. Browse some online immobiliers, to see what's in budget, and then research the area, or pose a question in the forum. At worst you can always fly out and take a look whilst we still have cheap airlines. My last trip to France was £2.04 (minus tax) from East Midlands Airport.