Cases are usually $15-20 by themselves. If I remember correctly, they came out with one for Days that featured Xion, Axel and Roxas on the clock tower and it was $25.
So yeah, I'd say so. :x Though if you don't like art, don't care about owning the cards yourself (you'll be able to read them from a computer image when someone inevitably scans them), and won't use the case... no, I doubt it is worth it. XD; But money-wise, yeah, it's definitely worth it.