All pricing is relative.
In other words, all prices will be judged by consumers in their relation to other prices. Your customers will compare the price of your product to the prices of other products. The big question is: what will they compare it to?
With expansion content, I always use the original game's price as my primary measure. Expansion content should not cost nearly half the price of the original game if it offers significantly less than half the amount of content.
But I don't assume that the majority of game consumers think like me in this matter. What do you use as your measure of fairness when considering the price of expansion content or full games?