Usually, FPC supplier recommends a set of standard FPC thickness values that they can handle without making any adjustments to their machines. Even though this can be customized, the customer must consider the following points while deciding on a custom FPC thickness:

Know the FPC manufacturer’s equipment capability: It is better to know the FPC manufacturer’s equipment capability; before the FPC design process. It is helpful to avoid the FPC redesign process.

High turnaround time: The customized FPC thickness will need some adjustments to the manufacturing machines. It will increase turnaround time when compared to the standardized FPC thickness. 

Additional cost: Yes, some additional costs will need to spend on custom FPC thickness because it may need extra work, material, and manpower than standard thickness FPC.