That might be cheap merchandise in 1982, but to Spock it's a finely made, decorative mirror that pre-dates the Eugenics Wars. You just don't find them in perfect condition, but there it was in an antiques store. He paid a pretty credit for it, too. He was probably out shopping with Leila Kalomi during his time on Earth. He was scooping up her cultural acumen, and she was trying anything to win his affection.
Too bad it likely didn't survive the destruction of the ship. Kirk could have taken it back to 1986, pawned it, and created another potential predestination paradox!