This will depend heavily on the type of matter (medical malpractice, legal malpractice, etc.) and in some types of matters, such as medical malpractice, the particular specialty involved. Also, the factual basis of the claim is highly important as the deviation in question may be readily apparent, or may involve additional research by the affiant. Overall, I doubt one could be obtained for less than $500 in any field, and the upper limit for a complicated time consuming review would be in the thousands. You cannot obtain an Affidavit of Merit on a contingent basis.
My comments on your inquiry are based on the limited information provided. If you have any additional information which you feel could impact my understanding of your situation, please feel free to provide. Nothing in my comment, including the comment itself, constitutes an attorney-client relationship. This is an informal advice forum for general informational purposes only.
Are you asking for a legal malpractice claim? The cost of preparing it is the least, as any expert must spend the time and energy to review all the facts and documents and do any research necessary to provide the AOM. There is no going rate. It is the amount of time (# of hours) x the hourly rate ($). Varies according to expertise and time and going rate for that professional. Impossible to give a "going rate".