Hi Nicole,
The weather in Christmas is sort of cold - I guess around 30-40 degrees or less. Especially in Kielce -- it's "the Polish capital of wind and cold"

. Kielce is on the East of Poland and is not as well-developed as Warsaw, but it's a nice city. In Warsaw it's a little warmer. It's definately cheaper to stay in Kielce than in Warsaw (especially if you're going to stay in a hotel).
One US dollar is currently worth about 3.10 Zlotych. A regular hamburger and fries in Mc Donald's may cost about 9 Zlotych (3 dollars) - note that the portions are a little smaller than in the US

. Actually, most things - especially foods in restaurtants - seem a little smaller compared to what is in the US where they put tons of food on your plate. But the quality matters, not the quantity

.
For one person to be comfortable I would assume to have at leat $100 a day.