Well our trip has now come and gone...(great trip! can't wait to go back!)
Long before I had posted the question in this thread, I had received quotes from Avis, Hertz, and Europe by Car.
The long and short of it is that booking last minute (we went with Avis, online, 2 days before we picked up the car) didn't change the price in any significant way.
Europe by Car's quote for a "Small economy" vehicle for 7 days was about $660.00 USD, and that was including a fee to pick up at the airport and drop off in a completely different city/country. (It would have been about $175 USD less to pick up and drop off at the same downtown location according to the quotes I had) I don't have the details of the quotes for Avis and Hertz handy but Hertz was more than Europe by Car, and 7 days at Avis was about $575 CAD for the cheapest car, airport pickup, and different country drop-off.
When all is said and done, we wound up picking up our car, a Citroen C5 (i.e. NOT small economy, but rather, a fairly nice sedan) at a downtown Paris location. We reserved it for 7 days and kept it for 12, without even telling them, because we could not get in touch with them (long story). We then dropped it off at a totally different location in Lille, at the train station (extra fee for that, as with airports)...
GRAND TOTAL....$756 CAD...for 12 days instead of 7...a nice car instead of a tiny one...
We hit 2 toll highways in France for a grand total of 7 euros and a tank of diesel cost us $100 CAD. We went through 1.5 tanks. The overall cost was reasonable to us and the flexibility of booking when we felt like leaving the city was very convenient.