First find the area of the first wall: 5 feet * 8 feet = <<5*8=40>>40 square feet
Then find the area of the second wall: 7 feet * 8 feet = <<7*8=56>>56 square feet
Then add the areas of the two walls to find the total area: 40 square feet + 56 square feet = <<40+56=96>>96 square feet
Then divide the number of square feet by the number of tiles per square foot to find the total number of tiles needed: 96 square feet * 4 tiles/square foot = <<96*4=384>>384 tiles
Then find the difference in cost per tile: $13/tile - $11/tile = $<<13-11=2>>2/tile
Then multiply the number of tiles by the cost difference per tile to find the total cost difference: $2/tile * 384 tiles = $<<2*384=768>>768
#### 768