Like everything, you get what you pay for. A $10 pair will likely have questionable comfort and optics. It's not as simple as just shading the sun. They do the job but not well. Lots of technology and development goes into a $100+ pair of sunglasses. Oakley and others actually invent new manufacturing methods, technologies, and machines just to produce their sunglasses. Those things cost millions. They've gone so far as a $4,000 (one pair) carbon fiber line.
It's understandable to not want to deal with the hassle of caring for a nice pair at work but for optics, comfort, durability and style, there's a huge difference. It really is like comparing a $20 pair of generic shoes bought at Walmart and a pair of Nike's or New Balance. Would you want to put a 10 hour day in wearing those $20 shoes?
Another angle based on experience is that if I buy a $15 pair of sunglasses, I'll have to buy a new pair every couple of months or less. The $130 Oakley's, for example, will last years and if you need, Oakley sells replacement lenses if you do scratch them so you can extend the life even more. So after a year or so, the nice pair costs no more than the 5 junk pairs you bought.