The local Gauls might have lost a forest, but I am sure there was no shortage of forest at the time. It would have cost JC nothing. The real cost was borne by the grunts who had to chop the trees down, move them, dig trenches and erect the walls.

The Roman army was very adept and building, and then tearing down, temporary fortifications.

Also I think I remember that the Roman soldiers had a general command to entrench if they had nothing to do. (Like a, you know, "take the broom and clean the floor if you got nothing to do" kind of rule)

Indeed. Roads, bridges, aqueducts, and many other kinds of infrastructure works was being done by the roman military when they weren't marching (i.e. stationing and "resting").