How much more diesel will I use If I leave my Air conditioning switched on?

Your A/C uses about 1/3 to 1/2 a horse power...Maybe a little more at idle...Just turn it on, and be comfy...your not even going to notice a difference..

Find the Mythbusters episode where they tested this.

Very little. The A/C doesn't run 24/7 anyway.

"I am willing to suffer personal discomfort in order to save a few cents' worth of fuel."

I have never believed anyone who said that, and I'm not going to start now.

Not enough to make that big a difference.to your pocket book. Opening the windows to cool down will use about the same amount because of the extra wind drag.

Well, it is up for debate but common thinking is that today's cars have such sleek aerodynamics that there is little difference between the extra fuel used to drive the AC compressor vs the extra aerodynamic drag from having the windows open.