Burlingame airport parking, which is attached to the SFO Hilton is probably your cheapest option. They're usually around $10 a day taxes and fees included, it is uncovered though and reservations have to be made online before hand.

As mentioned I'd also look to see if Costco has the coupons for Park N Fly, when I was last there I was pretty sure they had them, comes out to about $11 a day.

Worth mentioning that ParkSFO's prices do not include taxes and fees, which I would say are about an extra $3 a day on top of their listed prices, where as the SFO Long term price includes any taxes and fees. The SFO long term lot has become my new go to parking even without the discounts given how easy it is now to take the air train vs waiting on a shuttle.