Many would argue that the walls have already dropped in price too much.
At th7 and up, the cost of walls allows the game to slow down a bit and become a "grind". While many complain about it, the delay of gratification of being able to max walls for many is a huge feeling of accomplishment.
There is also a glut of resources at th9 as heroes are leveled up but all else is done. Toss em in walls.

