The price reduction was made due to "production optimizations" that allows the Model B+ to be cheaper to make than when it first launched. This version continues to sell well, even though a new model, the Raspberry Pi 2, launched earlier this year. The hardware inside the Pi 2 is six times more powerful than the Model B+ for the cost of just $35.
The company also continues to sell a third Raspberry Pi version, the Model A+, for the price of $20.
Source: Raspberry Pi