The core would prevent skin buckling and that is nice. But does the core cancel the low e to weight ratio? Balsa core plywood might have superior modulus of e to weight ratio. The Driggs DJ-1 wing was skinned with 3/64" plywood. I would guess an ultralight might use thinner skin, perhaps 2/64" (1/32"). Aircraft Spruce has 1/32" birch ply for about $2/sq.ft. I don't know what it weighs. Might be 3.5 pounds per 4'x8' sheet or .1 pounds per square foot.