Have you checked Warby Parker? They have reasonably priced sunglasses that are really cute... I don't know that you'll be able to get anything under $50 outside of maybe checking out Nordstrom rack, TJ Maxx for discounted designer brands? Fossil had pretty good eyewear but I believe they stopped making them, so you may be able to get some for a good price on their site if they are still trying to get rid of inventory.