It varies by course, so you'll need to look through the catalog. The prices are monthly, and all courses are "at your own pace", but they give an estimate for each course of how long the course will take.
https://www.udacity.com/courses
Intro to Hadoop and MapReduce costs $150/month (or $105/month early enrollment) for a course that is estimated to take a month.
https://www.udacity.com/course/ud617
Exploratory Data Analysis costs $150/month (or $105/month early enrollment) and is estimated to take two months to complete.
https://www.udacity.com/course/ud651
Data Wrangling with MongoDB is $200/month ($140 early enrollment) for an estimated two months.