I've been very impressed with Carbon Engineering's synthetic fuel, although a bit concerned their most recent investors are fossil fuel companies. In one interview CE reckoned they could produce synthetic, ie carbon neutral, fuels for about 25% more than the standard dinosaur smoothie.
If that held for air fuel it would add only a small amount to an air fare - 5% of a ticket price is fuel, so a $200 air fare becomes $202.5 - but air fuel does do more damage because of its release altitude, and apparently the lack of impurities in synthetic fuel might be problem for aero engines. I do wonder why Carbon Engineering don't start building many large plants, turn out fuel at 25% more and have governments drop their taxes as it's carbon neutral (they'll lose out on fuel taxes anyway with EVs). Even if the pump price is higher there will be enough of a market for carbon neutral (and less toxic) petrol to get the development of carbon capture plants going. I guess the question is, is that 25% difference realistic now, or what they're aiming for in a few years time?