The implants still have to be cheaper than the antibiotics.

The implants would likely be pretty cheap -- a one time purchase of an RFID sensor encased in glass or similar non-immunogenic coating. RFID tags are on the order of a few cents. Probably this type of sophisticated sensor is going to be more like a few dollars in quantity.

The reader and the software are going to be a few hundred dollars or maybe a couple thousand. But it's a one time investment, plus ongoing implants as the herd changes over time.

I suspect the main selling point is going to be the one-time capital investment aspects and the fact that antibiotics are being painted as an undesirable additive in meat and dairy. When the general public starts to dislike a substance, suppliers scramble to distinguish their products, e.g. "No added growth hormones" and the like.

So you're saying the correct answer is a massive tax on all antibiotics not dispensed in response to a human doctor's prescription for a patient that is seen?