While there certainly are differences between the carriers in terms of prices, here is a general breakdown. First, the carriers that offer free data often, very often, don't mention and certainly don't emphasize that the free data they are offering is 2G and not 3G of 4G. 2G speeds are OK for emails, messaging with Whatsapp but certainly not good enough for streaming, browsing (unless you are really patient) using Google Maps or Skyping. In order to have faster speeds, the carriers that offer "free" data are more than happy to sell it to you. Regardless of the carrier, faster data speeds are going to be expensive and typically very expensive.