Section 3--106. Sum Certain. (1) The sum payable is a sum certain even though it is to be paid (a) with a stated rate of interest or by stated installments; or (b) with stated different rates of interest before and after default or a specified date; or (c) with a stated discount or addition if paid before or after the date fixed for payment; or (d) with exchange or less exchange, whether at a fixed rate or at the current rate; or (e) with costs of collection or an attorney's fee or both upon default. (2) For the purposes of subsection one of this section "a stated rate of interest" shall also include a rate of interest that cannot be calculated by looking only to the instrument but which is readily ascertainable by a reference in the instrument to a published statute, regulation, rule of court, generally accepted commercial or financial index, compendium of interest rates, or announced rate of a named financial institution. (3) Nothing in this section shall validate any term which is otherwise illegal.