The highest in in the industry for API pricing right now is GPT-5.4-Pro, OpenRouter adding that as an option in their Auto Router was when I had to go customise the routing settings because it was not even close to providing $30/m input tokens and $180/m output tokens of value (for context Opus 4.6 is $5/m input and $25/m output)

(Ok, technically o1-pro is even more expensive, but I'm assuming that's a "please move on" pricing)