Your example is a bit complicated but

IF(ISERROR(FIND(“30_”,B27,1))==”TRUE”,D27*E$8,D27)

what I see is the "TRUE" is not correct and the 1 following B27 is suspect.

Try:

IF(ISERROR(FIND(“30_”,B27,1))==TRUE(),D27*E$8,D27)

or

IF(ISERROR(FIND(“30_”,B27,0))==TRUE(),D27*E$8,D27)

as the first character position of B27 is 0 not 1