Trouble with check engine light every time I drive the car on 1994 Mercury Grand Marquis

Check engine light keeps coming on. Every time I drive the car. Advised by the mechanic that the sensor light needs replacing What would the cost be for this.

Asked by for the 1994 Mercury Grand Marquis
Hard to tell which sensor they are talking about, But its usually the oxygen sensor and you can check that price here and get another opinion here