35 ILCS 450/2-20
The Department may determine the value of products severed from a production unit when the operator and purchaser are affiliated persons, when the sale and purchase of products is not an arm's length transaction, or when products are severed and removed from a production unit and a value is not established for those products. The value determined by the Department shall be commensurate with the actual price received for products of like quality, character, and use which are severed in the same field or area. If there are no sales of products of like quality, character, and use severed in the same field or area, then the Department shall establish a reasonable value based on sales of products of like quality, character, and use which are severed in other areas of the State, taking into consideration any other relevant factors.
35 ILCS 450/2-20