The knobs are "brushed stainless" plated plastic with plastic parts inside. It's possible Bosch is using higher quality materials, but this is the standard Bosch range knob that goes on all of their ranges, so I suspect it's made down to a price. They're injection molded parts that cost about a quarter a piece including parts and labor.

The important parts of the knob are all inside the range. But still can cost as little as about 10 bucks for an OEM part and 15 minutes of repair work if one of those should break.