This is a very interesting question, but it also seems like common sense. As mentioned earlier, the greater the amount of fill the more insulation a down garment will possess. As such, you would think that this would automatically make down products with a higher fill more expensive. But, this is not necessarily the case as other factors play more of a role than the weight of the down.
For example, the quality of the fabric and construction, in addition to the quality of the down fill, play more of a role in determining the final price. So, technically, a 650-fill power down vest could be more expensive than an 850-fill power down all because the 650-fill has a superior fabric and construction.