Hi 

Try the following code

class ProductProduct(models.Model):
_inherit = 'product.product'

margin_product = fields.Float(string='Margin',
compute='_compute_margin', store=True)

@api.depends('list_price', 'standard_price')
def _compute_margin(self):
self.margin_product = 0
for record in self:
if record.list_price and record.standard_price:
record.margin_product = (record.list_price - record.standard_price) / record.list_price * 100

Regards