I have a couple of photos of Costco price labels on my phone so I did a bit of math. To pay with a credit card as opposed to cash costs 2.3% more, so you're really getting 0.7% back on your purchases. In order to pay for the $450 annual fee, you'd have to spend 64,285 pesos per year or 5,357 per month - just to pay for the annual fee.
Edit: Mea culpa, I just looked at the price labels again and you pay the same as cash price if you use the Costco card, so you'd have to spend 15,000 pesos to cover the annual fee. Lo siento mucho!