According to My Small Obsessions, this watch is no longer in production. It was worn by Kate since at least 2005 (while she was still dating William).
It’s a two toned (gold and silver) timepiece with Roman numerals on the watch face.
Here it is on Kate:
She always wears her watches on her left wrist.
She seems to rely more on her pricier Cartier watch now that she is a royal. This particular model is no longer available for sale, so I could not locate an exact price. Tissot watches for women are priced at around, $300-$400, so I imagine this watch costs around that amount.