The well-known restaurants will be fully booked, so reservations should be made early. The larger restaurants host special dinners at premium prices with paper hats and silliness included on New Year's eve. You'll find them advertised in the Paris press a few weeks in advance. The smaller, more intimate restaurants may send the staff home and shut their doors tight. Christmas eve and Christmas day are quiet in Paris.
People are at home or with their families eating, drinking and celebrating.
Metropole Paris has great descriptions of past meals and seasonal events. At 58 FF this is the best overall deal, bringing the cost of a ride on the metro or bus from A to B to just 5.80 francs. Purchasing one ticket is 8 FF, so you've already saved 22 francs when buying 10 tickets at one go. Remember, Paris is a city where you will want to walk a lot to see, feel, breathe, live the city. Taking a taxi back to your hotel after pounding the pavement along with all your shopping is a good idea.