Hello Bionic_Leaf,
Simply put no, I guess you’re generally talking about render/performance cost. What material instances do for you is to help you streamline your material creation and tweaking process. Performance wise they are just an other material (draw call), and their cost depends on their parameters (i.e. texture size) and the (VS/PS) instruction count.
Here’s a link to unreal’s documentation on Material Instances: Creating and Using Material Instances | Unreal Engine Documentation .
If you have a bunch of Master Materials which are basically the same apart from differing parameter values, you can make your life way easier by creating one Master Material, exposing every parameter you need to be able to change, and instance the rest off of that Master Material.
Hope that helps. Just let me know if you have any questions, or want me to go into more detail.
Regards,
Vecherka.