If you do drive in the Express Lane, you can pay your toll online through an ExpressToll account or through License Plate Toll (LPT).
There's no single set cost for the Express Lane—your toll will vary depending on when you're driving, the type of vehicle you're in, and how many people are in the car. If you don't want to pay the toll, you can just drive in the regular lanes, but since they'll be more crowded, it might take you a little longer to get to your destination.