Okla. Stat. tit. 15, § 15-113: Contract void when consideration cannot be ascertained as

agreed.

Where a contract provides an exclusive method by which its

consideration is to be ascertained, which method is on its face

impossible of execution, the entire contract is void: Provided,

that where a contract provides an exclusive method by which its

consideration is to be ascertained, which method appears possible on

its face, but in fact is, or becomes impossible of execution, such

provision only is void.