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To calculate cost savings you need to know how long the lights will be on, and the savings will be, for whatever period you choose, (halogen kWh) - (LED kWh) * cost per kWh.
kWh = wattage * time * number of lights / 1000
6W * 5hr * 12 LEDs /1000 = 0.36kWh
50W * 5hr *12 / 1000 = 3kWh
If electricity is 20c/kWh, then you would have saved (3 - 0.36) * 20c = 53c.
However, you may need more than a 6W LED to replace a good quality 50W halogen, or else be prepared to work under lower levels of illumination.