There are a couple of gas stations within a mile of Hertz LAX (a cash or debit-card only Arco at 7370 La Tijera Blvd, just west of the 405, and cash or credit-card G&M at the intersection of La Tijera and Manchester) which are consistently cheaper than the pre-paid price, but the pre-paid price is very competitive with most other gas stations near LAX.

You might want to compare gas prices in LAX/Inglewood with prices where you will be traveling -- an app like GasBuddy.com is excellent in this regard. If prices along the road are lower than pre-paid and you have time before returning the car to fill up at one of the two stations mentioned above then it's probably not a worthwhile option.