Is this for a 7-passenger van? What days are where? Minivans are expensive. I assume you've looked into alternatives, such as a larger SUV or renting two sedans. You don't say what, if any, discounts you could sue and whether your price includes insurance. On a recent trip I used discount codes from Costco and from AARP, and in the past I've used codes from airline frequent flyer programs. I looked into Priceline and Hotwire but in our cases they were not less expensive. Are you arriving at SFO and then driving to L.A.? As noted, having a car in SF itself can be a liability, due to traffic and parking fees, even at hotels. Sometimes you can save money from renting at an off-airport location (e.g., downtown SF), though you will likely have a much smaller selection of vehicles to choose from (not sure if they always have minivans). One-way fees aren't always listed as such; sometimes there is just a higher (sometimes much higher) base fee. I found this to be true with Enterprise, for example (which is one company that doesn't normally allow one-way rentals except between airport locations).
I-5 is the quickest route to L.A. The part over the Tehachapi Mountains can be closed by snow, but that is usually for a fairly brief period of time. You might say why you want to drive vs. flying or taking the train (of course, you'd still need a car in L.A., even if you don't use on in S.F.).