I don't have dollar figures, but if you were to buy all new factory parts, it wouldn't be worth it. Best thing you could do is search the salvage yards for a complete used system. Problem with that idea is two things: one, they generally know what those systems are worth, and two, if the tractor is in a wrecking yard, the steering on it is likely shot!
To do this right you would need the complete steering box, a pump and reservoir, all the associated lines, 2 steering cylinders, different radius rods, etc. Whatever you do, don't fall for any system that simply adds an orbit motor to the column of the existing box for power. Believe me when I say it will not stand up!