I think (correct me if i'm wrong) the forester per slot, when upgraded, is equivalent to a level 7 building? So the epic is equivalent to 3*7 = 21 level 1 buildings (or 4 level 5 buildings). The footprint is less than 4 level 5 buildings. Additionally it can do other activities should your island evolve such that you need that, and it can of course be moved at a free cost.
It really shines when you start using it to produce coal or planks, as a single buff on it is more efficient than buffing 4 or 5 separate buildings.
As for the upgrade costs, yes, they seem high, but then again, many of us have so many excess resources or gold coin it is not really a bother. Perhaps it may be more accurate to say:
At your present state in game, you are not yet able to fully use the epic work-yard to warrant it.