Costco - Is it worth it

After some debate we decided to join Costco for a year. I had been with a friend and impressed by the prices on diary and produce and we felt like the savings on milk alone were worth the cost of a membership. The regular membership is $55 for a year but they have an executive membership that earns 2% back that is $110. 

While the executive membership is twice the price, Costco is so sure you'll use it (ie that you will spend at least $2750) that they will refund the difference if your 2% cash back check is less than the cost of your upgrade making it a no loose situation. $2750 x 2% = $55 Since the total doesn't include gas I highly doubt we will spend at least $2750 but at least we will get the difference back no matter what. I guess many people either spend more because they are getting 2% back or don't come back in for the difference. 

Here's a price breakdown of items I believe are a good price at Costco. I have heard that prices vary by store and can fluctuate. The gas is a great price often 10 cents cheaper but the standard gas here in Colorado is 85 Octane which is not good for your car. We are having to buy midgrade 87 Octane and Costco only sells 85 Octane and 91 Octane. The 91 is often the same price at the 87 at other stores but I wish they sold 87 at a discount. 

• Cheddar cheese- $2.50 per pound for a two pound block. I'm happy to get it on sale at Kroger for $4 per pound so this is a great price. 
• Bacon - $3/pound - $11.79 for four pounds.  I don't know if this is a good price since I've never really kept an eye on price per pound at the grocery store. 
• Breakfast sausage $2.33/pound 
• Butter $7.49 for four pounds - $1.89 per pound (awesome price!) 
• Fresh strawberries- four pounds flat at $1.09 per pound 
• Eggs - 18 count $2.99 - 16 cents each. What's up with egg prices? Oh my! 
• Protein powder - six pound bag for $49 - $8 per pound. Cheaper than what we usually buy online. 
• Milk - The Costco in Augusta sells the gallons individually but the Costco near us only sells them two in a box which is fine since David drinks a lot of milk. Two gallons for $3.95 - $1.97 per gallon! This is a savings of at least a $1 per gallon versus the grocery store. Milk is cheaper in Colorado than it was in Georgia. 
• Extra Virgin Organic Coconut oil - 72 ounces for $20- .256 per ounce. Fantastic price! 
• Dog food - I was feeding Lilly Iams or Purinia One but the Kirkland Brand is cheaper and much better quality according to Dog Food Review so I'm going to switch her over in September. 40 pound bag $28- .70 per pound verses the $1+ I was paying at Walmart for Iams.