The table to the right, created by PCmag.com, outlines the most popular headsets on the market right now.  As you can see, they range in price significantly and the higher end headsets are generally for experienced VR users or businesses.  As an online instructor, your students may have access to some of these headsets, however, most will probably have limited funds.   

The Google Cardboard headset doesn't have a lot of bells and whistles, but it gets the job done for a fairly inexpensive price.    The Google VR site offers all types of headsets ranging from $7.99 to $69.  All of the headsets on their site are Google Cardboard ready (they will work with the app).  Students can download the Google Cardboard app, put their phone in the Google Cardboard headset and begin their VR experience.