First Orcs do you have the weakest wall defenses (in a way) - but if used right by experience player Orcs actually have the best wall defenses - thanks to all the separate sections.
And yeah - your town walls are meant to work together with the palisade walls - here`s what they should look like:
http://www.reverieworld.com/screensh...lands/orc2.jpg
So lets figure out a way to fix this... try finding "Repair All Walls" button on your townhall, waiting 30 seconds, logging out, logging back in - and see if build Palisade Button comes back.
As for the price - humans need to build two layers of stone walls, while Orcs only have 1 layer.