Sure but that one time cost has a 10 year depreciation. A lifetime of chalk is endless mining because you go through it so quickly. You're constantly buying chalk. You're not constantly buying projectors.

>that I'm willing to bet a computer is the equivalent of hundreds of years worth of a blackboard with chalk.

As someone who grew up near a limestone mine, I think you greatly underestimate how much work it takes to get rocks from the earth. I've seen some of the largest machines in the world working there. Its an huge operation.