Speaking very generally, if two otherwise identical sleeping bags achieve the same loft, then the only difference between ones using 600 fill power down and 800 fill power down will be their weight. And the less loft that is required, the less this matters.

As for cost, I wouldn't concentrate too hard on fill power as a significant factor in how a retail price is determined. The labor and all the other materials are going to be a large chunk of the manufacturing cost, to which you must add transport costs, marketing costs, and profits.

In the end, temperature rating, workmanship, durability, cost and total weight are all going to matter. The fill power is only one fairly minor factor to consider, even though it has become increasingly visible in marketing terms.