I thought that the real thing was very helpful not only for the ability to get used to the CBT interface, but also for the detailed performance analysis compared to average. It would really take an excessive amount of time to do that sort of analysis yourself.
Also, it's good to have the latest questions/updates.
All in all, it seems like the question comes down to your financial situation. If you're really strapped for cash, then I can understand why it would be a tough call. In my case, I have enough student loans so that $100 in academic expenses won't even be memorable when it comes time to pay the loans back.