Hi There
Try
=IF(A1="","",IF(A1<100,A1+10+(A1*0.15),IF(A1<200,A1+20+(A1*0.23),IF(A1<300,A1+30+(A1*0.35)))))
First if brings back blank for blank (Use 0 for calculation field) then you can get 6 more IF functions in (Can do an extra 7 if you use a another reference field.