White with gray is definitely less common, as you noticed. How someone defines "rare" is subjective. I wouldn't call that rare personally, it's in that unique or harder to find category like maroon with gray interiors (85/86 only), both the black/saddle and white/saddle combo's, with white/saddle probably the least common or closest thing to rare. A little more common than your combo but still not as common is black/gray(especially 87/88's) and silver/burgundy, compared to black/burgundy and silver/gray combo's. If you check the SS registry you can get a very rough idea on the color combo's. Small sample size but enough cars in there that I feel it gives a rough % to these types of things.

-86'SS 383 CCC QJet- BRF 2004r-8.5" 3.42
-87'LS 350 MAF/SD TPI- CRF 2004r-7.5" 3.42
-81'Grand LeMans Safari Wagon 350 CCC Q-Jet/CZF 2004-r/8.5" 3.73
-07'TBSS Stockish daily driver
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