Break a leg; it will cost you an arm and a leg

Before the event in which I gave a talk, one friend of mine told me “break a leg!” I thanked her and thought a little bit on that idiom.

There are many opinions on the origins of this idiom according to different contexts and times. Above all, the most frequently explanations of the origin are the ones which say that it was superstition the reason for this idiom.

I could mention and give further information on the origin of idioms. But what I want to express here is not the origins of idioms or their uses. What interests me is the use of idioms among English, specially in Academic contexts.

I do understand that idioms are part of the culture and enrich the language.

But, how often are idioms used in academic contexts?
Are idioms a colloquialism use of language? Do people use idioms to avoid formality?

I have these doubts and some others. I will appreciate if you could answer some of my questions.