As what was pointed out by Sharky, SC has numerous times in the past changed prices to fit new content. If you are building level 10 or 11 walls, I'm sure you've experienced it already.
The way to look at this is just before you purchase an upgrade is ask yourself, is it worth what you are going to spend? Knowing that 10 minutes after you drop the loot that number could change. Doesn't matter if its a wall, a lab upgrade or a defence. If you don't want to spend the loot, don't click the upgrade button. While some players may not like that the walls got cheaper they also got the benefit of them after they purchased them.

