I'm not sure you're going to find a quality car cover for the price you're looking for. Buying a cheap one can do more harm than good to your car. Note that I said I'm not sure you will find a cheap one; someone on here who knows far more than I do may be able to help you locate one.
What's the Best Car Cover for Your Z?
I'd take a look at that though, just for some reading on what to look for and if its worth it to buy one at all for your specific situation. He recommends covercraft, but they are a bit pricey. If you're in a situation where you really need a car cover though, the price is worth it.