Yes, imagery is generally not cheap. Around 500 euro for the airport is unfortunately a quite normal price.
Did you try to contact the airport itself? Maybe the already own some imagery that they might allow you to use?
Good suggestion.
Most airports have aerial surveys conducted after some alterations to the field. These include multiple high-res photo's. You can ask the airport about those. They may or may not 'own' them, but they could give you the name of the photo survey company they used and you could ask them. They would normally keep these on file for historical reference. They may be a lot cheaper than a satellite image and a better quality. Make sure you get the lens type and sizes they used so you can properly adjust it for use without having distance distortion. Also make sure you tell them what you plan to do with the images. They may give them to you for free or a lower fee, but may not know you plan to use commercially(releasing scenery to the public is commercially even when free).
I used to work for a survey company as a rod-man. The file room was filled with surveys and imagery. Its a good place to look. In the US, everything in the survey file rooms is normally also available from the City, County, or State government offices they were filed with.
Traverse points, pk in disks, EOP's, EOS's, EOPT, Benchmarks, Benchruns, elevations, depths, locations of man-made objects, locations of natural objects, blueprint checking, storm/sewer placements, staking, silt placing, etc, etc.
There is a lot more to surveys than what it looks. The interesting parts were checking the foundations of a new building once the walls and roof was installed. If it was off by more than a few inches in placement or even thickness, the whole thing could be rejected by the owner, destroyed, and had to be rebuilt by the construction companies for free.
