The faster the Internet, the more data Skype will use. So it will use more data per minute for calls on LTE compared to say 3G.
According to Skype, it's 3MB/minute for Skype-Skype calls (calls between users) and 1MB/minute between a Skype user and a landline/cell phone.
Line2 passes about 350k of data per minute when connected to a landline or cellular number over wifi or 3G, and around 700k per minute when connected to another Line2 client. That's 500 minutes of talk time for 175Mb to 300Mb of data, respectively.
http://support.line2.com/entries/21063208-how-much-data-does-line2-use
Talkatone and Skype have 1mb of data per minute it looks like.