"best" is obviously very subjective, but...
Airport/Long Term parking at Millbrae BART is $6 per weekday, free on weekends. 24 days will be around 15-18 weekdays, so $90-108 total for parking, plus ~$10 return for the BART trip to the airport (less if you catch the bus, more if there's more than one person - although obviously you can do a airport dropoff/pickup for everyone other than the driver).

Not as convenient as the airport parking lots, but especially for a longer stay it's going to be by far the cheapest.