If you have chunks of the wrist falling out, you're about doomed. $3000 for a new stock for a boxlock, $4000 for a new stock for a sidelock.
If the chunks are small and the pinning and all-thread was done correctly by an expert, then that same expert would fill with epoxy with sanded wood from the gun mixed in. Upon hardening, they would fix the hairline cracks that still show with cyano-acrylate glue (crazy glue) that is applied while rapidly sanding with 800-1000 grit so super fine sawdust binds to the glue concealing the repairs.
Then recut checkering (or expand the checkering) to cover the repairs and you've got maybe a functional gun.
As a sidenote, I can't count on both hands the times I've seen repairs like above done so flawlessly that you wouldn't notice upon inspection. This is how scammers dump $8000 purdeys and hollands for $25,000 at gun shows. You don't notice until you shoot the gun and it potentially all comes apart. You only detect the masterful repairs by realizing the checkering layout is bigger than it should be, or redone to wrap around the wrist. All the clever ways to cut-checker over crack lines.
We need to see pictures to see how many chunks are missing and how big they are. I can tell you if it is salvagable, a workman like repair of this type will cost $500-$1500 and in the end, its often cheaper to just have the gun restocked for $4000.
This is why there are $1000 shotguns for sale everywhere for $200 with broken stocks. The cure exceeds the value of the gun by 10x and even once repaired, a repaired $1000 shotgun is only worth $400.