They are both correct but say slightly different things. "Is it expensive ?" is saying "Does it cost a lot to buy ?". "Will this jacket cost me a lot of money if I buy it now". "Was it expensive" is only referring to the purchase made in the past. "Did it cost you a lot of money THEN".

I like your new jacket. Was it expensive? Why? Because the person purchasing their jacket happened in the past. They already own the jacket. If you were asking about a restaurant you could use "Is it expensive" For example: Person 1: Have you been to that new French restaurant? Person 2: Yes Person 1: Oh, that's great. Is it expensive? Cultural notes: You wouldn't normally ask someone if their jacket is expensive though. That would be considered rude in north America. You might ask a close friend or family member but not an acquaintance.

I think you should use was because you are asking about the action of buying the jacket which happened in the past. You would use is if you were in the store with her, and he or she was pondering over buying it

Hi! I am a native English speaker. "Was" or "is" are both correct, but I would probably use "was" because I am talking about the purchase of the jacket, which happened in the past. Both words are fine choices, however.