It's a well known fact that todays glues are a lot stronger than the wood. You can take a piece of wood break it, glue it up and break it again, and it won't crack in the same spot. Try it. I know that for a fact becasue I have done it.
The whole issue here is that the gun should have been listed as having a cracked stock. Yes, there is a resurgence of 16 ga guns. They are bringing prices equal to 20 ga. guns. I do believe that in your description of the gun, being dirty and the stock being cracked, that $1,000 is high. But like Lowell said, if he's happy, we're happy?
Probably expect another $300-$600 for repairs and cleaning. What could you get now?