815 ILCS 405/2.8

"Cash sale price" means the price stated in a retail installment contract, or in a sales slip or other memorandum furnished by a retail seller to a retail buyer in connection with a retail charge agreement, for which the seller in good faith and in the regular course of business would have sold or furnished to the buyer and the buyer would have bought or obtained from the seller the goods or services which are the subject matter of a retail installment transaction, if the sale had been a sale for cash. The cash sale price may include any taxes and the cash sale prices for accessories and their installation and for delivery, servicing, repairing, or improving the goods.