It used to be you could plan to spend about $100 US per person per day. What would be a good round number to use in planning a 2 or 3 week stay in and around Paris in September/October (assuming the Euro doesn't go insane again).
Including hotel?
yep, hotel, food, etc.
So if that means two people combining $200/day, it can be done. You can find hotels for under $100/day most times of the year. For September and October, I'd reserve ahead and also check apartment rentals. Your food expenses are entirely variable. You can stick to grocery stores, crepe stands and couscous for a few days and splurge on others. Buy a museum pass and use carnets of Metro tickets and your other basic expenses are taken care of.