Alton Brown answered this question in his hallmark episode True Brew.
Regarding coffee beans or ground coffee: The various decaffeination processes all degrade the quality of the coffee to some extent. There are two ways the folks who sell coffee can address this fact: They can just live with it, resulting is a significant quality difference between regular and decaf (the decaf being inferior), or they can use superior beans for the decaf (Alton called these "gold" beans), so that after the impact of the degradation, the decaf is just as good as the regular. The companies that choose the former approach generally will charge the same for regular and decaf beans or grounds; the companies that choose the latter approach generally will charge more for decaf.
Note that this does not translate over to coffee by-the-cup. (Alton didn't actually address this...) it shouldn't surprise anyone that the cost of the beans/grounds is a relatively small percentage of the cost of coffee by-the-cup. Most of the cost is labor, facilities, utilities, and of course the cost of customer acquisition. Therefore, there isn't much driving different prices for regular and decaf.