Air refrigerant leaked out? on 2000 Nissan Frontier

The mechanic said that we would have to plug a leak because refrigerant does not wear out it leaks out. I was quoted $800.00

I haven't had them check for a leak yet. I just want to know how a closed system could leak and how much should they charge me.

Asked by for the 2000 Nissan Frontier
what exactly did the shop say was leaking?