Mountain bike brakes are extremely expensive, commonly costing 300+ for a full set of rotors, levers and calipers. But, you don't have to break the bank to get an amazing set of brakes.
The Magura MT2 and MT4 brakes are amazing brakes for your money. Specialized also just specced the brand new Magura MTS brakes that have a MT4 handle with the MT6 calipers. Another budget friendly mountain bike brake is the Shimano SLX. They are lightweight, amazingly durable and incredibly consistent.