Oh sorry, I misunderstood your Q. Anw, have you tried the Quantity Pricing Schedule permission under List of the Role record? If that doesn't work too, I'm thinking of creating a workflow (Set Field Display Type to Inline/Disabled for certain roles or users) but I'm not sure though if you impose it directly on the QPS columns (but for other fields, definitely) in the item record (unless you do not use QPS in the first place).