Costs of microcontroller with GSM module
It is possible to make a cell phone, perhaps not a smartphone, for a very low cost. For example, the Nokia 105 supposedly costs only $14.20 to make.  But as the article says, this does not include any software costs, licensing or royalty fees (see my comment under the question for more information on that).  For a company like Nokia, this could be a fixed cost amortized over millions of devices.  The GSM/RF parts are listed as around $5.  But you'll never be able to buy these chips yourself for anything close to that.

This article indicates the lowest BOM for a smartphone in China is currently $22.

Now when we're talking real smartphones, like the Samsung Galaxy S5 and the iPhone 5, things get a little crazy: the BOM for an S5 is estimated to be around $250, and the iPhone 5 around $200.  That is why they have to retail for $600 or so.