I've got 35 cards, less than 10% of the set, after years of collecting so this is not a realistic set build for me, but I still like to pick them up every once in a while and add them to my miscellaneous 1970s album (which also holds a near complete 1973 Calbee first series set and a complete 1978 Yamakatsu set which I finally found).
There is one thing about this set which I don't think the Engel guide captures which is the relative scarcity of some of the cards. Engel rates the set at R2 (fewer than 250 copies of each card exist), which seems about right for the easier to find ones. Having followed prices a lot on Yahoo Auctions though I suspect that some of them are short printed and much harder to find than the others.
For example, this card of Keiji Osawa, manager of the Fighters, is listed on YJA now for 29,800 Yen. Osawa is not a Hall of Famer and its hard to figure the rationale for that price. That dealer knows his stuff, I've bought from him before and he has good prices. If you browse his listings you'll find other cards from the same Nippon Ham Sausage set featuring big name Hall of Famers in similar condition for a fraction of that price. So its not like its some random guy listing stuff for ridiculous prices or anything.
There are quite a few other examples out there, like this card of Yutaka Enatsu for 100,000 Yen from a dealer who I've also bought from numerous times before (including some of the cards pictured in this post) and has much more reasonable prices on other cards from the set.
The only thing I can think of that would explain these cards being so expensive is that they are known rarities to Japanese collectors. Engel lists the Osawa as a common, and the Enatsu at what you could call the "medium level star" price, but this doesn't seem to jive with what those and some other cards go for in Japan.
The mystery to me though is how do collectors here know which ones are extremely high value and which ones aren't? There is no Japanese guide that catalogues these, Engel is the only one out there and his guide doesn't note any difference in rarity or value. SCM during its day never even tried to tackle this set. And there is nothing distinctive about the cards themselves either that would indicate they came from a given series or anything (something which the lack of card numbers also complicates).
One possibility that I am toying with is that the cards might have been regionally distributed according to team or possibly league. If you look at the 35 cards I have you'll notice there is an extreme imbalance. I have:
32 cards of Central League players
3 cards of Pacific League players (1 Hawks, 2 Orions)
Looking at Engel's checklist guys from teams of the two leagues look roughly evenly represented, yet the number of Pacific League cards in circulation seem to be quite a bit lower than Central League cards.
My collection isn't really big enough to draw any conclusions from of course, but when I look at YJA there is further evidence to support this. When I do a search for Nippon Ham Sausage cards, then organize the results from highest price to lowest, its really noticable that almost all of the high priced cards from this set (excluding the other Nippon Ham Sausage set from 1977 and the more recent ones) are of players from Pacific League teams (particularly the Fighters and Lions it seems), with only a smattering of Central League players (mostly Sadaharu Oh, whose cards are expensive because hey, he's Sadaharu Oh).
The Yutaka Enatsu card seems to be the only outlier as it pictures him with the Tigers, but there may be an explanation for that. Enatsu switched teams to the Hawks while this set was being made and is also featured as a Hawk on other, presumably later, cards in the set, so that card of him with the Tigers was likely short printed.
I'm not too sure what this means in terms of how the cards were originally distributed (I know they came in paper packs sold with sausages, I have one of the wrappers), but it seems the end result is that most (possibly not all?) cards featuring Pacific League players are harder to find than most (all?) cards of Central League players. But even that doesn't perfectly explain the evidence, since there are also some cards of Pacfic League players that aren't too expensive out there. Its still a mystery to me, but something is definitely going on with this set.
Oh, and OH MY GOD WHAT A GAME! Yay Ohtani!!