How much are unused oliver wendell holmes 15 cent stamps?

I currently live with a 68 yr old pack rat. After finally replacing his last tv, he had to replace his cable box and relocate it. In so doing he went through an old plate or something for a rayon microwave full of things. He found a pack of oliver wendell holmes 15 cent stamps...and a 1975 garfield pencil sharpened up to the date. After giving them to me i looked closer and found that 19 of the 24 stamps had not been used. the pack itself is old but still in really good condition. With the add for a Tom Sawyer Commemorative stamp souvenir page on the back (order form on the inside of the back), and the picture of oliver wendell holmes on the front with the $3.60 for 24 stamps on the front as well. I found amazon selling 2 with 1 used for $9.95, it also stated to beware of pack sellers...but i'm not kidding, i was shocked to find unused authentic 15 cent stamps let alone 19 still in the pack! how much are these things worth?