How to work out a stock retro
Cost per unit = cost price × free units ÷ (free units + bought units)
On 1 on 6 at £18.50 the customer receives 7 bottles and pays for 6. The free bottle is spread across all 7, so it's £18.50 ÷ 7 = £2.64 per bottle, a 14.3% discount.
Why divide by 7 and not 6?
Dividing by 6 gives 16.7%, which is the discount on the bottles paid for, not on every bottle that reaches the shelf. The retailer's cost per bottle sold is £18.50 × 6 ÷ 7 = £15.86.
Deals like 2 on 12
Use the same formula: £18.50 × 2 ÷ 14 = £2.64. It's the same discount as 1 on 6.
What it really costs the brand owner
If the brand owner supplies the free bottles as stock rather than as a credit, the real cost is their own duty-paid cost of those bottles, which is lower than the customer's price.