With the Boxster, you have some tough challenges if you are using it outdoors. You need something that will stand up to a little weather, won't scratch your paint, and won't leave fuzzy stuff all over your covertible top.
I've had many different covers and I strongly recommend the Cal Car Covers "Superweave". You won't stay under $200 (will cost you closer to $300), but you also won't regret getting a cover that won't meet all the criteria I listed. It sucks to spent $200 on a cover only to spend $300 later when the $200 one sucks. Ask me how I know.