Not really rare, anymore, though they were, for a while. SWFA’s evil-bay store has them listed for $66.44, which ain’t chicken feed. (Admittedly, I have not priced chicken feed, lately.)
The older, prettier, Shinier bluer ones marked for .243 and .308, and “Made in Japan” seem to have some desirability, which keeps their pre-owned price high. Of course, the pre-81 BLR mags are, indeed, expensive, with asking prices in the low- to mid-hundreds, so be aware of that, before buying a pre-81 BLR.