Good question.
When the 3904 hit the scene, it came with an 8-round magazine. And accessory magazines in the blister pack on the classic blue card were made in large volume.
Shortly after, the follower was changed (spring also? Unlikely but maybe) and capacity was upped to 9 rounds.
The 3906 did not stay in production very long. The 3904 did, and the Value Line Model 909 came in the mid-90’s and used the same magazine. In the year 2000, the Performance Center made the very first of the 952 pistols (the no-dash, Bangers exclusive) and this pistol used the same magazine. As a matter of fact, by this time the S&W accessory magazines were now packaged in a clear plastic box with a blue insert card and the 952 was even listed on the card as one of the capable models for this magazine.
Externally and fit-wise, there is no difference between the 8 and 9 round full size single stack 9mm magazine except for an added witness hole. I should also think (but haven’t tried) that the newer follower used in the 8-round magazine would allow it to run perfectly well with 9-rounds in it.
They come up on Gunbroker often. I never go to Ebay, but since they are not hi-cap, they are probably on there also.
What are they “worth?” Haha, rarely are they worth what a seller tries to list them at… at least in my opinion. I would suggest that $35 is fair, more if it is a sealed accessory magazine in original package.
As magazines go, I would not term these particular magazines as scarce, and absolutely not rare. S&W made MANY, and they made pistols that used them as late as 2008, possibly later.