Nope.

There's no reason to declare it. It's a used item and for personal use. I brought my ring in too and had no problems at all.

Does eTravel even ask about that? I remember it asking about cash, and the amount you could bring in was ridiculously small, but I don't remember anything about goods or jewelry. It's not for resale, so they can't expect you to declare it.

You don't have to wear it, but don't keep it in the original box with the price tag on it. You're pretty close to the 150,000 peso tax threshold. Still, just in case you get inspected, make sure you have a photo of the receipt. That way they can't inflate the value. Over the years I've brought two expensive rings into the country and never had an issue.

All the best with the proposal too. Hope it's a moment you'll both remember.

Never tell her how much you paid for the ring. She'll find out from the pawnshop, not from you.