The real cost isn't in the money needed to pay off farmers to not grow things, it's to the consumers that won't have their favorite foods available. Americans love their corn, rice, beef, almonds, pistachios, and avocados, all of which are very water-intensive crops.
The high agricultural water consumption is basically consumers saying "I value my beef more than my lawn", which is pretty accurate. We hear about residential water conservation (which, as you mention, is a literal drop in the bucket) because nobody's seriously suggesting banning meat, but given the choice between water shortages (as long as it doesn't get to the point where drinking water is endangered) or going without favorite foods, most people would take the water shortages. Chances are, even a significant portion of vegans would rather go with the water shortage if everyone was a vegan and it was no longer an effective way to signal cultural values.