The existence of free lunch Is there a free lunch or something beneficial but with zero price? Is air in an unpolluted area free lunch? If so, the sentence "there is always a tradeoff between benefit and cost" is not in general true. Is this correct?
The concept of a free lunch is a philosophical/political phrase (not an axiom or law in economics). It was popularized by Nobel Prize winning economist Milton Friendman

The concept depends too largely on how you define free to make a general statement about the concept. For example, while Oxygen is readily available for your consumption from the atmosphere at a cost of zero, Friedman argued there was always an opportunity cost involved (you could be breathing ultra-pure oxygen from an oxygen tank to avoid pollutants in the atmosphere that will give you cancer) so technically there is a cost.