cocoa is the main input for that and is subject to weather and crop failure, which - surprise - is why its' more expensive. however if you're talking about chocolate candies (not raw cocoa) it is indeed less expensive now adjusted for inflation. the problem is the quality of chocolate candies has reduced, so the equivalent chocolate bar is probably more expensive even though the similar one is cheaper.

ironically cocoa is a great example of my point though - it's not imported from china, so there isn't a huge cost reduction.