Usually it’s assumed that Costco is ALWAYS cheapest. I’ve found that Amazon.com’s “Subscribe & Save” option with the 20% off is actually CHEAPER than Costco in some cases.
Just as a reference, the Costco pricing is mainly out of the Fullerton Costco located in Fullerton, California.
Here’s a referral link so you can try Amazon Prime for free for 30 days if you aren’t already on it. Not only do you get free 2 day shipping (which often for me is actually one day since one of their distribution centers are so close!), but you also get access to a ton of REALLY good TV shows and movies.