According to New Scientist (12/10/13, p 10) the IPCC now sees geo-engineering as essential if we are to avoid catastrophic climate change. That was the conclusion of the recent Oxford Conference on Negative Emmission Technologies.
New Scientist's summary shows seven technologies any one of which could, if energetically implemented, remove at least one gigaton of carbon per year from the atmosphere. That's 10% of annual emissions.
The costs, for what are often theoretical technologies, are highly uncertain and fall in the range $10 to $2,000 per ton of CO2 captured. So the cost of removing one gigaton of carbon per year would be on the order of $400 B dollars per year. That's a lot by any ordinary standard. On the other hand it's just a quarter of the world's annual arms bill.
But money isn't the only issue. Some methods require physical resources such as gas-tight strata or land for tree planting and these are needed for other purposes too.