I'm thinking it's probably cos' HB seems more complicated (paint scheme and design wise) than centurion.
Just like how the diecast Patriot is similarly priced as the Mark 42 @$310, although its has no BD parts, and how the Mark V was similarly priced as the IM 2 Mark VI @$210, although it had virtually no accessoriess.
Something interesting to note, is that the 1/4 bust War Machine mark 2 is $15 cheaper than the 1/4 bust Patriot, even though they're essentially the same thing. So complexity of the design (in this case, I'm guessing it's Patriot's color combination) probably had a hand in the price too.