Book of Mormon Critical Text discusses "pieces" in Alma 11 and its biblical background; shows that "piece" [of money] does not mean a coin.
Book of Mormon Critical Text: A Tool for Scholarly Reference, 3 vols. (Provo, UT: Foundation for Ancient Research and Mormon Studies, 1987), 2:587n617
cf Gen 33:19 gesita “pieces of money” [KJ marg rdg “lambs”—not in the 1828 Phinney or 1979 LDS KJ]; Ex 37:7 gesha “piece” (of gold); Josh 24:32 gesita “pieces of silver” [KJ marg rdg “lambs”—not in Phinney or 1979 LDS KJ]; I Sam 2:36 ‘agorat kesef “piece of silver”; Job 42:11 gesita “piece of money”; Ps 68:30 ratse-kesef “pieces of silver”; none of these are coins.