I started with the third roof option build from the original post then removed two of the 1x10 Bricks from each building. To the building on the left I added in three 1x4 Bricks, one 1x3 Brick, one 1x1 Brick, and one 4x4 Plate (for the roof). To the building on the right I added in two 1x4 Bricks, three 1x2 Bricks, two 1x1 Bricks, and one 4x4 Plate (for the roof).
In 6P scale the left-hand building is still 25.5 feet on a side and 11 feet tall. (The overlapping roof is 30 feet on a side.) The building will now take 4 Hits to destroy. It will still be Cover if the roof is blasted off but not past the third Hit. In 6P scale the right-hand building is still 25.5 feet on a side and 14 feet tall. (The overlapping roof is 30 feet on a side.) The building will now take 5 Hits to destroy. It will still be Cover if the roof is blasted off but not past the third Hit. Also, adding the reinforcing plate to the roof resolves the issue of the roof shattering when it's picked up by the roof.
Doing some quick checking; a 6x6 Plate (x4) is $0.13 on Bricklink, a 1x10
Brick (x10) is $0.29 per, a Large Jumper (x4) is $0.01 per, a 1x4 Brick
(x3) is $0.03 per, a 1x3 Brick (x1) is $0.03 per, a 1x1 Brick (x1) is $0.01
per, and a 4x4 Plate (x1) is $0.02. That's a total of $3.61 per building.
More quick checking; a 6x6 Plate (x4) is $0.13 on Bricklink, a 1x10
Brick (x14) is $0.29 per, a Large Jumper (x4) is $0.01 per, a 1x4 Brick
(x2) is $0.03 per, a 1x2 Brick (x3) is $0.02 per, a 1x1 Brick (x2) is $0.01
per, and a 4x4 Plate (x1) is $0.02. That's a total of $4.78 per building.
So both of these options are cheaper than most of the options from the original post and look a little more functional as well. I 'll leave people with one last size comparison photo.
I hope this helps people working on constructing buildings for their battlefields.