What is the value of jfk life magazine?

It depends which magazine you are asking about. John F. Kennedy was in Life magazine on several occasions. What determines the value of a magazine is the condition it is in, the date it is from, how rare that edition is, and whether it's about a person or a topic collectors care about. Without knowing which Life magazine, or the condition yours is in, I cannot give you advice; but I can suggest you visit a site like eBay, and see what price similar magazines are selling for.