I like the idea of stacking up a lot of 1 meter cubes of carbon into nice shapes. Each cube is just over 2 metric tons of carbon, or the equivalent of 8 metric tons of carbon dioxide. 20 cubic kilometers per year is enough to get us back to pre-industrial levels in no time, and build some interesting monuments while we're at it. A roughly 4-kilometer tall pyramid, anyone? Low low price of 2 trillion dollars each.