I did not look at the details, but it appears that neither of these estimates take into account opportunity costs. Typical farming profit is around $200 per hectare per year, so if instead you sequester 5 tCO2e per hectare per year, that would cost ~$40 per tCO2e, ~2 orders of magnitude more expensive. By the way, I believe $300 billion divided by 205 billion tons carbon = 750 billion tons CO2 would be $0.40 per ton CO2.
I did not look at the details, but it appears that neither of these estimates take into account opportunity costs. Typical farming profit is around $200 per hectare per year, so if instead you sequester 5 tCO2e per hectare per year, that would cost ~$40 per tCO2e, ~2 orders of magnitude more expensive. By the way, I believe $300 billion divided by 205 billion tons carbon = 750 billion tons CO2 would be $0.40 per ton CO2.