I'm not an economicsfags but basically, you are right. Not just dairy farmers but others farmers who send goods to CAN would be affected. I can't imagine it would cost anywhere near close to 200Bil to subsidise he farmers.
That's my way of thinking as well. We would be filthy rich as a nation. Or, even like Potus is pushing for. Bilateral, equal, fair trade. That's if Congress doesn't pass a measure to block Potus tariffs.
An idea is if the US can not consume as much dairy as we produce, then govt can help some farmers transform to another more prosperous crop.