The best that I can recall, the only good pair of sunglasses I have owned during my lifetime was a pair of Serengeti driving sunglasses that acquired with Budget Rent-a-car points I had earned. These glasses lasted about 9 months before finally breaking them, after sitting on them one too many times.. That was actually pretty good for me.
While there are many stores to buy cheap sunglasses from, I currently purchase mine from a store called "El Paisa". Paisa means "of/from Antioquia or its capital, Medellín". Apparently the owners are immigrants from Columbia.
Above the name El Paisa are the words "Muchas Ofertas" (Many Offers) and below the name are the words "Mas Barato Imposible" (The Cheapest Impossible).
If you look at the doorway, you will see a rack of sunglasses on the left side which are sunglasses while on the right side they are reading glasses. Each column on those racks is clearly marked with the price.
Here is a photo of my current selection of sunglasses.:
The top two sunglasses in the photo cost me a $1.25 each. I think the bottom pair cost me $2.00, probably cuz they had a "Ray Ban" sticker on them when I bought them.
This store is located on Luis Cordero just south of Sucre.