Middle-Tapped Transformer Full-Wave Rectifier Vs. Full-Wave Bridge Rectifier
The cost of the diodes is one thing. 

The cost of adding a center tap is another. 

Increased efficiency by losing a diode drop is another thing. 

You can get 24% more DC current out of the same weight and size transformer with the 4-diode full wave bridge. 

So if the filtered DC voltage is more than 2-3V, the full wave rectifier comes out ahead, assuming the diodes-vs-centertap is a wash.