Making a rough estimation, you should not spend more than $60 or $80 on your GMAT preparation
(including the Official Book, one or two additional books to reinforce one specific Verbal sub-section, and some "real" computer-adaptative GMAT exams for your practice).
Therefore, never (ever) spend money on those expensive courses, academies and books. This is my recommendation.
On my blog you can find more information on this,
All the best and good luck with GMAT!!
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More: "All I wish someone had told me about GMAT beforehand"
There are many things you want to know before doing the GMAT exam (how is exam day, what to expect, how to think, to do's...), and you have them in this blog, in a simple way