I think spending a maximum of 25 dollars for sunglasses is pretty reasonable. I would not pay more and I do not care about the brand. Some of my friends only buy them if they have a certain name on it.
I would spend $1. That's how much they cost at Dollar Tree. And Dollar tree is where I go to buy my sunglasses. I buy 5 at a time and put them everywhere. They have reading glasses too, for the same great price. They are light duty but who cares, if one breaks, toss it and start another pair.
For a pair of sunglasses, I would spend up to $10 only if I really like the design. But for prescription glasses, my budget would be around $40 to $50.
In my opinion spending over 200 dollars on a pair of sunglasses is too much, but of course it depends on how much money you earn. If you are an hard worker there is no shame in spending extra money for a gift for yourself. Peronsally I've never bought sunglasses paying more than 100 dollars, and it's still quite a lot compared to how much I spend for clothes and accessories. However I really need high quality sunglasses to drive during the summer, go to the beach or even just go for a walk when the sun is burning, and there is a huge difference between some cheap 10 bucks glasses or a designer, high quality pair. My favourite brand is RayBan since they are both stylish and comfortable.
I spent maybe a bit over 100 dollars, which in my local currency would be 3 times more. I spent that on Michael Kors glasses that I really like. I also have some by Calvin Klein but those were a bit cheaper, I don't intend on spending a lot of money again any time soon.
I've been known to spend a lot of money on shades, up to £200. I've never gone beyond that, though. I especially like Ray Ban and Giorgio Armani and have owned both, but sadly lost them at different stages. The last pair I lost was the last one I had at the time, a gorgeous Giorgio Armani pair I couldn't replace. Trouble is, I now ca't afford them and have to settle for much, much cheaper ones. I don't really mind, as long as my eyes are protected. But of course, if I had the choice, I'd up the budget **sigh**.