We have been to
Disneyland Paris and also to Paris at Christmas. But not DP at Christmas. I think it would be nice.
There are busses from Charles de Gaulle airport, where most US planes land, to DP. The busses are frequent and clearly marked. You will be able to find them. Just walk up to one, they will put your luggage underneath, and pay.
We went to DP for a daytrip from the airport when we had an unexpected 8 hour delay in a flight. A great way to spend the waiting time! DP is about 15 minutes from the airport by bus. Do not take the RER (suburban rail) back into town and back to DP. Not worth the hassle and the trouble.
As to cost - - I would stay at a hotel in Paris and choose a good day to go out to DP, taking the RER out (about 45 minutes and cheap). Our family of 3 stays in a charming traditional style Paris hotel, nice beds, baths, and balcony overlooking the street (once we looked over Notre Dame) for about $70 a night total, including breakfast. There are even better bargains in the winter, and the dollar is strong right now.
There is so much to do in Paris at Christmas. The department stores are like nothing in the U.S., and the decorations stay up until Jan 6. You will have warm sunny days (in the 60s) and also some rain. We have never found it cold. On rainy days, we go to museums, which are almost empty that time of year. One rainy day we went into Notre Dame and listened to a choir of nuns practicing. No crowds, and Paris is beautiful and romantic even in the rain.
But -- on a nice day, go out to DP.
About food prices, we find prices in restaurants on the left bank to be cheaper than WDW. We do not eat in Michelin rated places, but nice neighborhood places with great food beautifully presented. $12 - 15 buys a very good 3 course meal with table wine. You can find for less, also more. We also like to shop the grocery stores for many meals and spend no more than at home. In DP, we only ate lunch and I do not remember it being any higher than WDW.
I have looked at the DP hotels and think the price is high (figuring at $10 per point). Sometime I would like to splurge and stay one or two nights, but would not stay for an entire vacation.
Have fun! Paris and Disneyland Paris and Christmas would be triple magic