In French there is no single word for "cheap" or "inexpensive". Instead, there are several other ways to express the same idea.

les vols ne sont pas donnés. -- the flights aren't for free.
les vols ne sont pas bon marché. -- the flights aren't cheap.

If you want to say "inexpensive", here are some other ways to say this:

peu coûteux - "low-cost" or "affordable."
abordable - "affordable" or "within reach."
économique - "economical" or "budget-friendly."
à prix raisonnable - "reasonably priced" or "at a reasonable cost."
pas cher- this is a more casual way of saying "not expensive" or "cheap."

Colloquially, I think ‘ un bon marché ‘ is more likely to be used in the positive sense to indicate a price is good or reasonable, while ‘ n’est pas donné ‘ is a common expression in the negative for something ‘ not cheap ’, as in on the expensive side . 

Not too different in meaning and usage to the English expressions ‘ a good price ‘ or ‘ not being given away ‘