Of course you DON'T declare anything like this to the rogues at Thai Customs for this situation, no matter what their law says.

Frankly, you are asking for trouble taking a $30K ring on this sort of trip in the first place. Why don't you put the Real Thing in a bank safety deposit box at home, then get a cheapie replica to give to her at proposal time in Ko Samui, followed by an explanation that she'll get the real replacement on your return home. Most women worth proposing to will be perfectly happy with the sentiment expressed by the faux ring, and will understand that it's not a great idea to be carting around or wearing something so valuable under these circumstances. Remember that even if you get the ring on her finger, she's still got to keep up with it through sightseeing, beachtime, etc. Seriously, babysitting the object will become the focus of the trip, rather than focusing on each other's company.