Explanation of the flow/features: 

First, at the creation of a product, it should be added to a pricelist in order to apply the minimum margin on it. It's possible with an automated action, the screenshot below shows how to configure it: 

This will create a specific line (pricelist item) in the pricelist "Public Pricelist (EUR)" for this product so you don't have to update your pricelist each time you create a new product.

The pricelist items are configured like that :

• Apply on : products
• Compute Price : Formula
• Based on : Cost
• Price discount : - the margin you want

One it's done, you will need to display the product cost in the pricelist item (form view). To do that, you just have to add a related field that points towards the product>cost .  I called it PrCost. Make sure this field is stored, it will be necessary for the next steps.

It's now time to create the automated actions that will update the "price discount" field in the pricelist items. Here are 3 screenshots of the automated actions created (minimum margin for cost below X€, between X and Y€ and above Y€, in the order )

Once it's done, the price discounts are automatically updated when you modify the cost on the product template.

To resume : 

• Creating a product will automatically create a dedicated pricelist item for it, and based on what you put in the product cost when you create it , it will automatically have a minimum margin applied.
• Each time you modify the cost of a product, the existing pricelist item for this product will be updated.