The issue is that this price doesn't scale. Let's say that the current price of an offset is $5/ton-of-co2. Let's say that presently all offsets are being put toward replacing inefficient wood-burning stoves in third-world countries with much more efficient electric stoves, because it's the cheapest way to reduce carbon emissions. Right now, only a very small amount of offsets are being purchased (relatively little carbon is being offset); however, if lots of offsets were to be purchased (lots of people trying to offset their emissions) then we would run out of wood-burning stoves to replace with electric stoves and people who sell offsets would have to finance a more expensive type of carbon-reducing project, increasing the per-ton cost of a carbon offset. As demand for offsets rise, that new more expensive project would run out as well, and we would have to move onto increasingly expensive projects and the price of offsets would continue to rise.

In other words, if the whole world were trying to offset its carbon emissions, the price per offset would rise dramatically.