I can not find a reference sources that defines the minimum value of a major contract.
What is the definition of a major contract?
I have found a couple of reference points that I believe answer your question. They both have the same answer. However without knowing exactly which agency you support, I strongly recommend you check your agency’s guidance to ensure it coincides with the below information. These templates can be found under the DAU Acquisition Community Connection (AAC) portal. The templates are entitled "Acquisition Strategy ADDM Template v 2.4" and "Technology Development Strategy [or] Acquisition Strategy". Both documents state the following:
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Major Contract(s) Planned Guidance: For each contract with an estimated total value greater than $100 million dollars ($40 million dollars if during the Technology Development Phase), including all options.