The cost of an unlocked unit is $300 to $600 LESS than that of other devices with comparable specs, so mobile carriers should be able to offer the Moto G to the masses for hundreds of dollars less than any iPhone or high-end Android device by Samsung, LG, etc.
Mobile carriers could offer the Moto G profitably at a negative price -- for example, zero money upfront plus an instant $300 coupon rebate if one commits to a two-year plan. Or they could offer it with much cheaper monthly bills than economically possible with other comparable phones -- for example, 25% off one's monthly bill if one commits to a two-year plan.
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Edits: added context and examples.