Hallo. Not sure what you mean when you ask "what size"- do you mean what ratio, or what brand, or what? As for size, you need the type that fit a JK axle- either a JK Dana 30 or JK Rubicon 44 front, and a JK Dana 44 (Rubicon or non-Rubicon). Make certain you specify JK ring and pinion, or it won't fit.
As for cost, I pay $300 per axle. My guy is pretty good, and stands by his work. He takes his time, and sets them up right- Mark W.