From the aspect of average price of the eyeglasses and contact lenses, the walmart will offer less price than those in the costco. However, this is not absolute. If you have already had a favor on one type, you may respectively go to see their online store and check the price.
Personally, i prefer to buy contact lenses at costco? But i never make a comparison where are cheaper. If you want to get cheap contact lenses, you shall first make sure what type of contact lenses you want to buy. Only if you know what you want to buy, then you can search it out in diiferent shop such as walmart and costco. Now, you can make a compassion where are cheaper for the contact lenses. Besides, walmart or costco, you can also have a look on other contact lenses shop such as 1-800 contacts in order to make the buy the most worthy contact lenses.
Yes, I have some experience in buying glasses and contacts from Walmart actually. Technically, Walmart is the largest retailer in the world and its vision center is pretty complicated. They have got various types glasses of different brands. There are a lot of options from which to choose. Personally, I would recommend Walmart to you, despite that Costco is also preferable. Anyway, you should pay a visit to see which is better.