I don't think it's that straightforward. Stuff like planting trees should have the cost/ton go down over time due to scale (there's a land question but there's a _lot_ of federally owned land). A lot of other stuff could work similarly.

Mass production is a thing that brings costs down, so some of the carbon offset programs could take advantage of similar effects.

You're definitely right that some of these projects aren't infinitely scalable though. There are only so many gas stoves used in northern Uganda to be replaced..

The costs may go up as tree planting is more limited, but the more money in the ecosystem will allow new innovations to be discovered!