Here is what I have learned over the years, some through here and some through friends and contacts in the bat making industry!!
1 - Cost of the clefts (G1-G4 to Vellum) are pretty close to each other, when you look at overall cost of materials for the product, from the suppliers. We all know volume discounts exist so more you buy the cheaper they are.
2 - Cost to manufacture a bat is/should be a constant (including wasted materials etc.), assuming consistent manufacturing process and overheads, for each manufacturer. I.e. some makers' cost is X per bat and others is Y. What I am trying to say is if I am good/consistent at making bats and it costs me $60 to make a bat, it would $60 for G1 and $60 for G2 and so on for all grades + raw material cost.
3 - Selling price of a bat is not relative to the cost of raw materials + manufacturing cost+profit. So if it cost you $100 to make a G4 bat and $120 to make a G1 and you sell G4 for $200 you are not going to sell G1 for $220/240 i.e. making the same profit amount or margins points. Reasoning for this? you can put a label on it...
What is the difference then? well i don't believe people when they say all pressing is the same or optimal for each cleft, lower grades fetch less money and generally, in asia, people with lower incomes buy these bats and expect them to last long so the pressing is hard, hence the performance difference...people with bigger budgets and less care for money will buy higher end bats for looks and performance so the makers can actually press the bat properly.
Grading only comes into play of a bat performing when there are a lot of knots in the willow other than that i think its simply cosmetic. If a batmaker was worth his/her salt and had a freedom to make a good bat with good performance without the fear of an ignorant customer, complaining about damage due to use or longevity of the bat, they could make and press a bat in any grade that performed well. Only exception would be natural variance in the raw material where no matter how much attention you paid to the wood in making and pressing the bat some just would never be as good as others.