The sturdy material means that even a cheap pair will last forever. Just ensure that they are 100% cotton (or close enough) and have a good zipper.
Togs can look bad at the best of times but once the fabric stretches they are destined for the bin. Invest in the perfect pair and make sure you are paying for quality, not just a brand name across your butt.
In the past I have paid stupid amounts for surf brand thongs (dammit City Beach) but it turns out my $5 Target thongs have lasted way longer than a $30 pair.
I often hear people suggest buying cheap-as-chips sunglasses because they always end up broken. I've dropped my sunglasses on the ground numerous times (and my phone, and my ipod...) but they haven't broken because they are good quality. Once you've replaced your $20 wayfarers five times, there's $100 that you could have spent on a nicer pair. Most importantly: sunglasses need to protect your eyes so look for a pair that have at least a category 2 protection rating. Oh and btw, squinting gives you wrinkles.