You can determine how much your Christmas Barbies are worth by having the dolls appraised. The appraisal can be done through a local collector or professional. The value of the dolls will factor greatly on the condition.
2 Additional Answers
Christmas Barbies in the box and in mint condition can be worth a couple of hundred dollars depending on the year they were introduced. To get an idea of value you can check out Ebay.
The amount that your Christmas Barbies are worth depends on the model and condition of the Barbie dolls. There are many doll price references at places like dollreference and collectdolls.