There could be an even cheaper scheme for climate change mitigation that we haven't thought of (like stimulating plankton growth with iron, for instance).

And it's actually possible that the total cost of allowing climate change to progress unmitigated may actually be cheaper than 50 cents per ton of CO2; creating more fertilizer for farmland and building seawalls may become cheaper 100 years from now than the future growth-adjusted value of the proposed carbon-based tax credits.