While creating a product, it will require a unit price as well as that is a required field in the products and quotes tool, maybe what I can recommend is seeing that it is a case to case basis, you and your team can consider setting it to $0 first as a one-time frequency and once you do know the price of that particular service you can then edit the price accordingly.
You can manually set it for one service and in the future, a similar deal comes along with the same price you can then pull in that existing product.