All that you did in calculated dimension remains there.
In your expression you use field RATE_MULT. You can't refer to dimensions by label.
Use column(N) function or just add if statement in your expression:
=([Unit Price Paid]*if(isnull(RATE_MULT),1,RATE_MULT))*[Quantity, lbs]
Also, consider replace nulls with 1 in script as you don't use them:
Month(EFFDT)&'-'&Year(EFFDT) as MonthYear,
FROM_CUR as [Currency Paid],
if(isnull(RATE_MULT),1,RATE_MULT) as RATE_MULT
(biff, embedded labels, table is Sheet1$);