The metal shooting kit is, if memory serves, the 150 PK. The 150SK is the one in the large box with a target holder and shooting games book.
The 40% valuation in the Blue Book is probably from me. It's been my experience that the metal shooting box certainly has value but not that much. It's really only of interest to hardcore Crosman collectors. Otherwise, nobody really wants one. So, when a collector finds a nice one, they usually aren't looking for another one. The exception being the Montgomery Wards version which is a bronze color on the outside. The Wards version is a good deal rarer than the silver one. But, even then I wouldn't add much more than 40% again to a Wards version 150 complete with the Wards PK box.
As I recall, there are at least two versions of the Crosman silver PK box. The earlier version has a large paper label that usually gets trashed over time. The later version has a small metallic label that usually stays in pretty good shape. I would think that a pristine example of the early paper label version would carry a premium.
My view on the value of Crosman items, which is largely reflected in the pricing of the Blue Book, is that there are two considerations: One is rarity and the other is desirability as a shooter. It's really only when both factors are there that a Crosman model value starts to go up. This is best seen in the pricing of the different models of the model 160. The early versions are valued less than the third model, because it's the third model that is so desirable to shooters today.