One of the dirty truths about "sports drinks" is that they are sugar-laden, artificially colored, and chock full of calories. Some of them don't even have much in the way of the electrolytes you're trying to replace in your body.
Setting aside the discussion of whether you need one or not - that depends heavily on your training intensity and duration - you really don't need to buy one. You can make your own, cheaply and effectively.
This article on webmd will give you all the information you need:
That drink is cheap, free of artificial coloring, free of HFCS (High fructose corn syrup), and quick to make.
Personally, I customize it further. I don't like the taste of so much sugar, so I use about 1 tablespoon of sugar and 1-2 tablespoons of organic honey. I used Brita-filtered water, and I use either sea salt or table salt alternatively. I've sometimes added a sugar-free no-calorie blueberry antioxidant extract for taste and for the antioxidants. I sometimes add L-Glutamine to mine (to ensure my body has enough L-Glutamine to keep the immune system strong post-workout). I've used it as a carrier for Creatine, which replenishes the muscles better when accompanied by carbohydrates like sugar.
But the basic mix is fine. One warning is that this mix does taste a little salty, especially if you drink it without having worked out hard. But if you've really pushed yourself, the salt is exactly what you need and your body will crave.
Hopefully if you choose to use a rehydration drink this will help you - it's much cheaper and you can find your own ways to flavor it.