"Yours is way too expensive."
Translation:Le tien est beaucoup trop cher.
Perhaps you didn't spell "La Tienne" in the feminine's form: It is gender specific. As a UK speaker I have a problem with "way too" I imagine it is a slang term introduced into US English and Tien is a slang term introduced into French. A strange marriage!! I have always used it when I give something to someone. "Tien" meaning "It's yours" or "it's for you"
Le tien, La tienne are possesive pronouns, not verbs so the imperative doesn't come into question. Used in conversation, whilst handing an object to another person "tien" is sometimes used, without an article and translates as "yours" or "here" like in the English "here you are (it's for you)". For example, when someone hands you an unexpected treat and exclaims "tien" if you are surprised by the action. Language that consists of words and phrases that are regarded as very informal and are more common in speech than writing is described as slang "way too" and "tien" would probably not be used as written English or French, neither is formal, but are often used as spoken the spoken word. I am English and try to share what I have experienced in France. This use of tien was common in a group of my French friends when giving something to someone. Slang.
I was educated in England and I have not been to America. However, I am quite shocked by the attack on the every day speech of most Americans. We all understand what is meant by way too expensive. The intention of the words are clear and are in fact so common place I'm sure some of us (who have not been to America) have used the expression from time to time.