Robin,
You are right. This is why I specified the bit about having a KT67a to start with. The above is still probably the least cost option but calculating the exact cost will require an avionic shop to give you a quotation, unless you are PFA (like me) in which case you can DIY. The cost of mounting a transponder in a non metal aircraft which has no spare room in the panel and no existing aerial will be very high. The real world costs I was quoted from a number of avionic shops was much higher than the CAA estimates in the original consultation, which was another nail in its coffin.
Rod1