Sold listings on eBay can tell you more but with a quick look and out of experience as I have sold Elgin and other Pocket Watches, I would say that after some polishing of the covers, inside and out and a careful surface cleaning of the movement, you might get $50 to $90. A good servicing would help the price but the costs of having it serviced would likely be as much or more than what the Pocket Watch would fetch at auction or anywhere else.
At auction in my area I normally see Elgins go for at least $50 plus....I would research Ebay like Huntsman said and see if you can find a similar one that sold. If I were you I would list on ebay with a high starting auction price and work my way down if it didn't sell. If it is a sought after watch the market should decide its value with a week long auction. Good luck!