How do you track your costs?
You measure your cost of goods compared to your sales. That's a direct cost in providing the products.
You measure your labour compared to your sales. That's a direct cost in serving your product.
You need to measure your marketing against the sales that it creates as a direct cost of bringing in new guests.
Many restaurants base their marketing investment on total sales generated. You need to measure the marketing dollar spent than the sales generated as a result of the marketing.
It's true; it's challenging to track every sale back to a marketing strategy, but what you can track is an incentive. When an incentive is redeemed, you can ask if this was the guest's first visit. Now you can track how many new guests came in as a result of your marketing.
Here is the formula.
(New guest vale times new guests that dined with you)
Divided by the marketing cost
Anything over 1 means your marketing is producing more value than the investment is costing.