Thanks for the comments Jim. My understanding is that the cocoa percentage listed on a chocolate bar wrapper (European chocolate and dark chocolates here in NA) includes both the cocoa solids and cocoa butter. Dark chocolate usually is 65-85%. The proportions of cocoa solids and cocoa butter vary which is why you get different flavour/texture even for different chocolate with the same cocoa levels. Typically the cocoa butter is removed and then partially added back to adjust the taste and "mouthfeel".

Unfortunately Hershey's bars don't list this number, so I've had to get it from Google (11%). This may be wrong but it seems to be a widely quoted number, and makes sense as the minimum required by the FDA is 10%. However it might just be the cocoa solids, as you've pointed out.

Assuming that's the case, we can estimate the butter component from the other direction. Cocoa beans are 54% fat (cacao butter) and 46% solids (cacao), so if solids are 11% then there is at most 11% * 54/46 = 13% cocoa butter. I doubt Hershey's adds back this much butter, but anyways.

Current prices from Wigmore Wholesale:
Cocoa liquor (solids) wholesale around USD 5850 / metric tonne
Cocoa butter wholesale around USD 7350 / metric tonne

43g * 11% * USD 5850/1M = 2.8 cents
43g * 13% * USD 7350/1M = 4.1 cents

So still not a huge factor in the price of a cheap chocolate bar, and probably even less than that.

(That's probably more than I ever wanted to learn about chocolate!)