The Material Pricing Model settings govern the cost and sell rates for plate material.
If a plate is using anything other than material thickness based pricing (ie plate template pricing or individual plate pricing), it will be indicated by a warning icon, and hovering the mouse cursor over the plate will display a hint describing where the plate' sell price in particular is derived from:
The plate context menu has an option Edit Material Sell which will take you to the appropriate place to edit the sell price (user permission dependent).