Just got back from Prague.
The Christmas markets are on Starometske Namesty (large one but mostly food stalls, the setting is the nicest as the Old Town Square set up for Xmas is gorgeous, do try the Prague ham roasted on a spit before you), Vaclave Namesty and Havelska. The Havelska is the one I liked best and it was slightly less crowded.
Locals told us that this winter is proving to be exceptionally mild, with temperatures about 5C above usual averages, and I can confirm that.
Restaurant prices in Prague are still quite moderate. If you go to simple Czech style "restaurace" and stick to beer it is really hard to go above a 15 euro per person tab (my fav in this category is the Vojvanov Dvur in Mala Strana). If you like game and want to dine in style, try the U Modre Kachnicky in the Old Town (about 30 euro per person with wine, have the hotel book it for you)