On the back side, you could add a siding, which would let you park a second train. When switching is done with the first train and it goes off into the rest of the world, you can bring in the other train for more switching.
As for cost, I have read that it can be at least $50 per square foot. However, if you rebuild, al the locos and rolling stock is reused, the buildings are reused, and much of the track, especially the turnouts are reuseable.
If you enjoy it, it is worth every penny.