why are silver proof eagles vary in price ie, 1992 $104 while 1991 is $31 is it simply scarcity?

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  2012-W Proof American Silver Eagle $1 NGC PF70 PF-70 Ultra Cameo First Releases $109.00 1d 14h 4m

  2006 P NGC PF70 20TH REVERSE PROOF AMERICAN SILVER EAGLE COIN $408.00 18h 22m

  2006-P NGC 20TH ANNIV. REVERSE PROOF 69 SILVER EAGLE BEAUTIFUL SPECIMEN $214.62 16h 37m

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