I hate to tell you, most spa manufacturers cut costs to the absolute bone when it comes to the cover they provide with the spa. It tends to be an item that's rarely mentioned during a sales presentation, and almost no consumer asks about it or for that matter, can even tell the differences. We make, or have made, covers for a variety of spa manufacturers. For the most part, I'm not willing to put my name on the covers that they spec and purchase from us. Let me stress though, the covers we make for them (spa manufacturers) are to THEIR exact specifications and are not (usually) anywhere close in quality to the covers we offer directly to consumers.
Most original spa covers that come with a new spa have a life of about 2 years. A high quality cover should have a life of 5 to 8 years. Some good suggestions have already been made as to how you can care for and extend the life of any spa cover.