R, 51 bytes

function(s)sum((utf8ToInt(s)-48)*10^(nchar(s):1-1))

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Multiplies Utf8 value of each character minus 48 by powers-of-ten from the length of the string minus one, down to zero, and outputs the sum.

R, 51 48 bytes

Edit: -3 bytes thanks to Robin Ryder

function(s)(utf8ToInt(s)-48)%*%10^(nchar(s):1-1)

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Multiplies Utf8 value of each character minus 48 by powers-of-ten from the length of the string minus one, down to zero, and outputs the sum.