Udacity has around 200 courses that are completely free (but do not offer a certificate).
Udacity’s Nanodegrees, a micro-credential exclusive to Udacity is completely paid. These Nanodegrees can range from $199 to $2400. A few Nanodegrees are on a monthly subscription, but the rest of the Nanodegrees are fixed price and term based. Few of the longer Nanodegrees are split into multiple terms.
Each term has a fixed start date and end date. If students do not finish the projects before the end date, you get a one month extension. If students still fail to finish the projects in the extension period, they lose access to the content and have to sign up again (by paying the full price) for a new term.