We have a Costco membership. It costs us $40 a year to be a member. We buy certain items there that we know are cheaper than our grocery store. One item I buy costs about $22 at Costco and its $28 at Amazon. We also get gas at Costco and their prices for gas are usually the cheapest in town. You can usually save 5-10¢ per gallon at Costco.
But when I look at most of the items for sale at Costco their prices don't often seem that great. They have GOOD prices for sure, but I don't think they're usually the BEST prices.
Looking at a few semi randomly selected items, lets see how Costco compares to other sources:
Nikon D300 with 18-200 lens. : Costco price $2239, Amazon price = $2149
Both prices reflected a $300 instant rebate from Nikon.
Lexmark #16 inkjet cartridge : Costco charges $30.75, Office Depot was $29.99, Amazon.com was $27.28
Costco and Amazon both had free shipping. For Office Depot you would have to spend $50 total to get free shipping.
Huggies size 2 diapers: Costco has a pack of 258 for $52 or $0.20 each., Amazon's price is pack of 192 for $34 or $0.177 each
In all 3 cases Amazon.com was cheaper than Costco.
Of course there are bargains to be found at Costco. Bulk food items and personal items are often cheaper at Costco than anywhere else. Costco does have consistently good prices. But don't assume they're the cheapest or even that their prices are better than other local retailers.