https://www.businessgreen.com/news/4060842/landmark-achievement-rolls-royce-easyjet-hail-successful-hydrogen-jet-engine-test-net-zero-commodities-hub
I'm not sure how much extra, but I would definitely pay extra to fly on green hydrogen. Probably not a percentage. More like $100-$300 regardless of cost of flight (so maybe 100% extra on a cheap flight and 20% extra on an expensive flight).
As it is right now, greenhouse gas emissions is more a limiting factor on my flying than cost, but at a certain cost, that would invert (and as the cost of flights these days rises, cost is becoming more of a limiting factor).