There is no “extra cost” for the customer to have an FAT performed on their system as it is generally included in the scope of the project. That being said, the expense that is factored into the project for FAT is dependent on the customer’s requirements. It is very important for you to communicate early in the project exactly what your testing needs are so the criteria can be outlined and agreed upon by both parties. This helps sets your expectations to the manufacturer and they will be sure to include the relevant tests in your checklist.
While FATs can be costly for the manufacturer with respect to the time and labor involved in performing them, the risk of not doing an FAT or doing a poor one are usually seen during start-up, and at that point the effort needed to do to rectify the issues takes even more time and labor and often must be done without shop support, so the labor is not as qualified. In short, it’s more time and cost effective to do an FAT and do it right to mitigate any issues prior to delivery.