Does anyone know what this ? Letter opener is worth.

Looks Japanese in design and white or transparent jade on the handle. You might google jade handled letter openers

I agree, looks like white jade & brass. You might also check with a museum or reputable antiques dealer.

I would contact a local appraiser, or search online. There are experts willing to offer opinions from photos. Also, search eBay; I have often found objects that were exactly the same as my object. One was a clock that I was convinced was rare, very old and sure to fund my lavish retirement. Imagine my disappointment to find it was built from a hobby kit in the 1940s. Understand that eBay is not a reliable source for valuation of objects, (you'll notice that the same item may be listed for sale at many price points on any online trading site) but it will give you a general idea of the availability and approximate value. Good luck!

RESEARCH E-BAY

It's Chinese. Most were not marked unless it was a designer like Tiffany. The jade is actually a belt hook. Can't tell if it's silver, brass, or what. You may want to have it checked out by an appraiser or auction house that deals in Chinese collectibles. You can even check out a Chinatown museum. The little research I did price can vary from $50 to upwards of $500 +.

What is this piece's history - do you know? Did it belong to a relative, possibly somebody who acquired it during WWII, or are you of Asian descent and inherited it from a relative? You have to be careful of knock-offs/fakes. If you live in New York city, I would contact the Asia Society and Museum and see if they have somebody on staff who could examine your piece, ditto the China Institute, also in New York city. Unlike some antiquities dealers or possibly some appraisers, they would not have any possibly ulterior motives to offer to buy your piece for a low price when it could be valuable; but be prepared you may also get an answer that tells you it's just a post WWII Japanese knock-off of a Chinese antique.