Cooksey & Shugart publish a pocketwatch and watch handbook with prices annually. If you go to your local Barnes & Noble, you can probably see from the book right off the shelf. While I'm not familiar with Zenith, your example looks to be pristine. Lots depends on the number of jewels in the movement, complication, face or dial condition, if it's all original, runs and if the case is gold filled or solid gold.
Since the movement has a serial number, you can write Zenith and they can probably give you some history behind your pocketwatch.
