Basically to cut a long story short The Lumia 520 has recently been launched in Walmart selling for the equivalent of ?96 which is $150 and has already sold out. Like the link below states, this may be the move for Windows phone to grab a decent chunk of the market share. It only has to take one phone, and the 520 is a good phone.
Microsoft goes mainstream to win phone share | Reuters
Microsoft goes mainstream to win phone share | Reuters