I don't think that there is that much of a direct correlation between property prices and broadband speed. I think its to do with the fact that people earning higher salaries are more likely to be able spend money on entertainment which requires high speed broadband. In the Europe and the UK there has been a big shift to video-on-demand and think its the demand for these services from people already living there that has attracted the broadband services in the first place. These where already relatively high priced areas. Once you have these services, when you look to relocate, you check that the area you relocating is going to offer the same or better. Which it usually does because those areas have already attracted the broadband investment because of the consumer demand.
Bear in mind though, that they talking about a different market to SA. In the UK for example you can check the available broadband services for any area online.
http://www.uswitch.com/broadband/postcode_checker/
Can you go onto the Telkom website, or Afrihost website, type in a post code and pull up a report of what level of broadband services are available ?