is a job offer valid if the salary stated is "what the employer thinks the potential employee is worth?

In a strange way, this is probably an offer that, following acceptance, would lead to the formation of a valid contract. The obvious issue with the offer is that a material term is left to the discretion of one of the parties. Normally, a contract would fail if a material term were indefinite and there were no means of resolving it. But this "indefiniteness" can be unilaterally resolved by the employer.