The plaintiff agrees to supply the defendant's building company with 2 tonnes of cement. During the negotiations the plaintiff agreed to supply at the price of $ 800 per tonne. The sales contract, prepared by the plaintiff, inadvertently states that the price is '$800' rather than '800 dollars per tonne'. When the plaintiff signs the contract he does not check that the price stated is per tonne. The defendant company seeks to defend a claim for payment of $ 1,600 on the grounds that the plaintiff is bound by the document signed to accept $ 800 for the cement.

Identify the nature of the mistake, and then determine if it is operative and state the consequences for the parties concerned.