If you tell your network analyzer to report s21 magnitude in dB, then that is the free space path loss in db's between the two antennas.
To figure out received power, it helps to convert the transmit power into db form also. Say you had 2 milliwatts transmit power. That is 10 LOG 2/1 = +3 dBm transmit power. You need a power meter to actually measure the power going into the transmit antenna's connector.
Then from your network analyzer you know the free space path loss is 30 dB.
The received power is simply +3 dBm -30 dB = -27 dBm.
-27 dBm is 2 microwatts.