Assuing they were kept in a good dry condition - and you wanted to shoot them, I'd probably offer him $ 2.50 a box for them. Depends if they're field or target loads ( 9's, 6's or whatever ..)
If you are looking to buy them as a " collectable " - it'll depend on the brand and the condition of the box - the better the box, the more the value, maybe $ 5 to $ 10 a box if they're an old winchester or remington box.