There are no makers marks. They are only stamped "Sterling."

Thank you for contacting Mearto with your appraisal inquiry.

It is a beautiful set of what you normally call glass holders. Such vintage silver glass holders are being offered at auction on a regular basis. The type and style is often seen in Russian/Russian silver, however the marks on Russian silver are different. They have a lovely design with the beautifully chased and cut puttis and flowers. The same goes for the lovely etching of the glasses. You can tell that a lot of handwork and effort has been put into making these glass holders.
They are made of sterlingsilver and stamped sterling. The use of sterlingsilver has been known since the 13th century, however it was not until the late 19th century that you began stamping with the word Sterling. This means that the cup holders were made in the late 1800s or early 1900s.
It is hard to tell the country, but Germany or the Netherlands could both be possible countries of origin.
As for the estimate - the price you have seen may have been for a russian pair of silver cup holders - as your particular type of sterling cup holders are not that unique nor expensive - unfortunately.

The estimate provided is for a well-advertised specialised online silver auction, where I believe the glass cup holders has the best chance of realising its fullest potential. However, there is no measurement for the sentimental value you might attach to your grandmothers cup holders. Retail "asking prices" can be higher and vary.

Please let me know if you have any additional questions. I’m happy to help.
Thank you