Isn't there a cheaper option, I think that you can connect between the AUX connector of the CD32 to a serial port of the Amiga 1200, but I think that you need a signal level in the connector as the voltage from the AUX of the CD32 is 0/5V, due to it being a keyboard connector type, and on the Amiga 1200 it is +/-12V.
Regards,
Adam