PRICE
It is difficult to estimate the price at this point. Materials should be more expensive than those used for hot air balloons, since they have to be multilayered. Even if they are ten times as expensive the price should be under 10/ m2 (see http://www.alibaba.com/showroom/hot-air-balloon-fabric.html). To build 20 m diameter chimney would require as much fabric as 300,000 m2 of fabric or $3,000,000. Outside roping, mushroom cap, ground base, labor and machinery to inflate will increase the price. However, I do not see it bringing above $5 million.
Thus, if 25,000 chimneys of such size are enough to stop global warming, the investment will be less than 1,250 billions (annual US budget deficit). Furthermore, as we get more experienced with building super chimneys, we can make them more powerful by making them taller and wider.
The benefits will be millions of acres of arable land in deserts around the world, fresh water, electricity etc. It is quite possible that we can end up making profit on solving Global Warming problem.