They aren’t expensive here locally; it is crazy you can fly 4000km for 10 euros.
Not the point though really; where does that (expensive enough) money go and is the airline taking care of their environmental obligations? Is the gov? Nope. Low/no tax on kerosine and greenwashing. It has to be more expensive and with clearer laws to make it work for the world.
Had friends over who paid 9 gbp for a return ticket London-south Spain on Ryanair. I see rows of tickets even in the summer for 9-30 euros for the same trip on different budget airlines.
If the price were E50 for that flight, it would still barely make a profit if you pay for the carbon credits to offset; 1 gram would cost 0,00007 (I see the current price for 1 credit for the EU is around 70 euros); a 737 has about 115gr per seat exhaust per km as per Wikipedia , so that makes the price for a 4000km trip around 30 euros only on emissions per seat. And that’s with all seats filled (it will use a tiny bit less with less seats but not that much).
And this is not including the contested and highly unfair lack of kerosine tax which competing transportation (cars, trains, buses etc do have to pay), but that’s another story.
Tickets under a 100 return means that someone is cutting corners with the environment. At least until we have fully neutral/clean planes (proven), which will come eventually.