Good idea. Walls are too cheap in EB considering the historical effort involved in constructing them.

For me, your prices seem a bit too high, though. I would make them:

Palisade: 1600 - 4 turns to build
Wooden Walls: 3200 - 6 turns to build
Stone Walls: 9600 - 8 turns to build
Epic Stone Walls: 25600 - 12 turns to build