Finding Wilson Current Mirror Output impedance
Some ideas:

I haven't used P-spice in a while. One way to solve this problem of measuring current in other spice programs is to insert a 0 ohm resistor and then that allows you to get the current measurement.

Another thing that might be messing with the calculation is the voltage source. If I remember right a voltage source in spice has infinite impedance, so that might be throwing the calculation off. A current source would probably be a better source to use in the calculation.