Hmm, in a world of $400 LionChief Plus locos, $18 per 4x8 2" foam sheet does not seem expensive. In any case, you do not have to fill the entire space under an elevated section. For instance, you could raise a 2" thick foam surface on 5" risers to reach 7" in height. The risers could be wood, foam, or some other material. I might use strips of the 2" thick foam on edge, for a 2" wide riser of any length up to 8', saving a lot of foam and conserving your $$.
You might consider accessing an internal area layout area using Lionel's 30" Lift-Out truss bridges as has been discussed in other threads.
To use Woodland Scenics inclines with O gauge, I will sometimes use a pair of inclines side-by-side to get a greater width under Fastrack.
