It really depends on a large factor of issues. First you have the scale issue, then after that, which aircraft, who made it, and finally how many people are willing to spend an x amount of money.
A good example is Pan Am, which is a more highly sought after airline compared to others in 1:400. You have the many A300s made by Pan Am Model Company, those can often be found on retailers for as low as twenty bucks (I guess due to the high number of them, there were around 14 different A300s with runs of about 250 each I would guess.) Then you have Gemini Jets or Pan Am Model Company 747-100s, which can often run from the low 40s and upwards on Ebay.
So really, there isn't really a "cheapest airline" to collect out there, it mostly comes down to individual models. There is also the question in 1:500 scale about the landing gear, particularly herpa. If it is the Old Generation gear (big rolling gear), typically they cost considerably less these days than the New Generation (scaled landing gear). In 1:500, Lufthansa could be one of the more widely produced airlines in that scale though, mostly due to the cooperation with herpa, so it would not be as surprising to see the market filled with 1:500 scale Lufthansa, and in return these often being cheaper than other airlines.
My Airport: Nowa Warszawa Int'l (NWL):
Attempt at first model airport (2005ish?)http://www.freewebs.com/nwlairport
Hey, model plane manufactures, how about some Braniff PZL 230Fs.