Hi AmanKidCACS,
First off, a 690 is a fantastic score (it's just a bit below the 90th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. Depending on the Schools that you plan to apply to, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
ask-admission-consultants-124/GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function, your Official results and CAT score results show that you essentially performed the same way each time (+/- a few points). If you do choose to retest, then you might want to think about this whole process as more of an investment in your future than a series of expenses to be avoided. The cost of adding some additional study materials will likely be less than the cost of retaking the GMAT - and the long term benefits (including getting into a better School, potentially receiving Scholarship money and all of the income that you will earn over the course of your lifetime) make these type of investments something that must be considered.
GMAT assassins aren't born, they're made,
Rich