retail price != manufacturing price, so say just 2-300 bil
Freak spikes in demand cause freak spikes in supplier parts prices. Manufacturing volumes of complex goods are quite inelastic. Burst manufacturing price would likely be much higher than steady flow retail prices, not lower. Zuckerberg would basically have to redirect humanities entire smartphone manufacturing capacity for multiple years. Including the part that usually ends up sporting some fruit brand, this wouldn't be cheap at all.
$300 * 3 billion people, so $900 Billion give or take which coincidentally is right around the market cap of the entire company.