I am using the rpart function (in the rpart package) in R to fit a cost-sensitive classification tree. I have read the corresponding vignette, which says that misclassication costs are incorporated via altered priors (section 3.2.2 on page 8). However, I am not sure that I understand exactly how these altered probabilities affect the splitting criteria/impurity index. Does anyone know? 
Altered priors for classification trees
classification;decision trees;cost function