Why is that more expensive?
In terms of complexity, implementing DP vs HDMI 2.1 is not materially different. They both have fixed rate links, packets, Reed-Solomon forward error correction, DSC, etc.
But IIRC DP is royalty free, and HDMI is not.
You can check that there in this presentation (https://www.vesa.org/wp-content/uploads/2011/01/ICCE-Present...), page 32 and 33.
The majority of DP traffic is still brute force video data, interspersed with heavily packetized secondary data.
Over the years, I've spent many hours wading through DisplayPort data debug traces, and I've always wondered what people were smoking when they called it 'packetized like Ethernet'. It's just not true. (And FWIW: even old HDMI can transport secondary data packets just the way you can with DP. It's how audio-over-HDMI is done...)