December 25 and 26 is public holiday in the Czech Republic, so some shops are closed this two days. But do not worry a lot shops and markets are open even during holiday.
Price for eating out depend of place you choose and also how far it is from the main tourist road. In the city center you will get dish for 7-10 EURO, but 3 tram stops from city center in Zizkov you can get dish for less than 4 EURO.
While pretty much every major Czech city (i.e. Brno, Ostrava, Olomouc, etc.) has a very nice Christmas market, nothing tops the markets in Prague. The biggest is at Old Town Square, and, in my opinion, the nicest is at Namesti Miru (it just seems cozier for some reason). The market at Namesti Republiky is a bit gauche for my taste, while the Wenceslas Square market (down at the Mustek end) is a bit too manic.
If you haven’t been to Prague yet, don’t worry about the Czech locations – the city is surprisingly easy to negotiate with a half-decent map.
In Brno, you can visit the Moravian capital’s market (situated in Namesti Svobody, or Freedom Square). If you rent a car it’s about a 1.5 to 2-hour drive from Prague. If you plan on using another mode of transportation it’s better to taking the train. If you don’t take a EuroCity train it’s a bit cheaper than the bus, but the extra cost of an EC train isn’t so bad, and it’s a much more pleasant ride.
If you decide to go to Brno by bus, then definitely go with Student Agency – there’s a toilet on the bus, and they usually show a movie (in Czech, but you may be able to request English subtitles) and offer snacks.
The train to Brno can take anywhere from 2.5 to 4 hours (approximately) depending on whether you get a EuroCity train.