The easiest way to start air combat is against civilian vehicles, equipping that charger laser in your stores to the Aircar. The Seagull missiles are meant to be pricey - all missiles are, but they get better when you can manufacture them. As such, the Airspeeder is really a stop-gap measure to field looted missiles if you can't manufacture Hunter-Killers yet, and it is often better to wait until you can get true interceptors. That being said, 300,000 is the high end of selling off everything (including hostages) from a civvie vehicle, and military craft only go up from there. Think of using those missiles as a trade for possibly looting better guns (and maybe more missiles!) from light military craft, such as runabouts and cutters.
Anything bigger than those targets, you really need an interceptor that can take a real hit. It's perfectly acceptable to wait to get into the air game until you have more advanced equipment and have enough manufacturing power to make worthwhile the economic gains from shooting down craft that don't land. Raiders, Light Gunboats, and Military Transports are all good targets once you start seeing them, as you can get more missiles and guns from them, and have useful equipment for the tactical game. You can take any of those down basically once you have a Hunter-Killer with a single full set of missiles, two light weapons, and one other interceptor with any weapon. Even the Hunter-Killer alone is enough, but more firepower means less repair time.
