A person breathes 20-22k times a day (although I agree that number will vary greatly among people).
2,000 gallons/day is about 300ml in/out per breathing cycle. My estimate is that you breathe in/out at least one liter, recall that total lung capacity is about 6 liters so about 20% of that seems like a reasonable estimate.
I chose to do 2-3 liters * 1.225 grams/liter * 22k times = ~50-80kg. That's a bit high now that I think about it, but no way is as low as 9kg.
Edit: Chat says a healthy adult averages 500ml per breathing cycle (honestly surprised is that low), so, final estimate ~13-14kg if that is true.
(I like metric units but you can do the conversion to burgers per squared eagle and compare)
Edit: Back of the envelope math puts it around 60-70kg per day, conservatively.
Can I just stop being right?