Permissions to restrict buying price list visibility

Using ERPNext v12. Roles are assigned to internal and external staff with restrictions on the external staff to prevent them from seeing Item Cost / margin info. I need to set permissions to restrict outside sales users’ roles from viewing Standard Buying Price List, while allowing visibility of Selling Price Lists. I haven’t been able to achieve it via Role Permission Manager or User Permissions that won’t drill down to the dependent area. I’m willing to pay a bounty.

R Bradley