Hey Sixgun,
I guess if you want to accept the going prices on internet gun auctions, you might find an example of this Savage auto model to gauge your son's purchase.
If it is the Savage Model 750 Browning style auto, takedown, 12 gauge, 4-shot mag. I would say it is worth $200-250. With adjustable choke...$275.
Gun Trader's Guide.
Jim