That's a custom gun....not a "restoration" by any stretch. And the restock is not nearly so well done as the seller implies. A custom gun is not a commodity item....it's essentially a one-of-a-kind or, at best, one of very few that are similar. And the right price will be whatever the buyer and seller agree on. The only way you can say a custom gun is "not worth $xxxx" is if you can buy or build it's equivalent for less $$$.
Something else to consider....you can pay 10-15% "too much" for a decent Fox sub-gauge today....hold it 5 years and you can break even....hold it 10 years and you'll make money.
To duplicate that gun for $5,000, you'd have to find a project-worthy 16ga AE for < $2500. Not impossible of course but, IMO, not likely.