Why do you say that? Typical fuel consumption values for passenger aircraft are 2.5-4 liters per 100km per passenger. So if you fly 1,000km, you'll use 25-40 liters of fuel. At current prices (around 60 cents per liter), that's $15-25 worth of fuel.

A liter of jet fuel contains 35-38 megajoules of energy, which is around 10 kilowatt-hours. Assuming 5% efficiency of using CO2, water, and cheap solar electricity (3 cents per kwh) to synthesize fuel, the cost of input energy per liter would be around 60 cents, which is the same as current fuel prices. The actual cost would be higher because you need to pay for the plant, workers, consumable catalysts, transporting the fuel to airports, etc. But real world efficiency would likely be higher than 5%. Also solar panels are still getting cheaper and more efficient, so 3 cents per kWh may be considered expensive in a decade.

Even without electric aircraft, there's no reason in principle why aviation needs to be expensive or bad for the environment. If demand for petroleum causes prices to increase enough, synthesized fuels will become economically competitive.