How much could a slot that shows up at most twice per year for ~20s, for ~2% of web users, be worth, and where does that sit in the market? It sounds tiny, to me.

Yeah it sounds tiny to me too. I strongly doubt that many people will change their choice after the first time so the only way it's worth as much to Google is if they think they can keep the market mostly as anti-competitive, which the government is indicating they should not try.