info on Beckhoff products

What is the typical price difference between a standard and a customized version of a Beckhoff product? (e.g., 1. operator panel with additional serial interface, 2. additional software function)

I don't understand the question. It sounds like you are asking about their "Control Panel" hardware. Your local salesman can quote. Re software, many people write the HMI in VB.net (or C#.net) so if you already have that plus Windows, just labor cost.