I've never taken this particular GRE class before but I'm interested in the guaranteed part. I mean if for some reason you don't score that amount do you get your money back? If so then maybe it's a good investment.
I'd call the company and get some references to call and talk to in person.
I took a kinda expensive course too and in the analytical part I did my best but didn't really improve a ton ("studies" show this is typical). A lot of people were like, "see, you wasted all your money!", but the piece of mind I got over preparing so much and knowing I did everything I could was well worth it.
A lot people I know say a private tutor is better than classes.
Whatever you decide, best of luck to you.