There is no statutory minimum. The investment must be substantial in proportion to the cost of the business and sufficient to make it operational; a full-price purchase of a small established company is the strongest form.
Measure the investment against the business, not against a rumoured threshold
Hypothetical example: The proportionality test compares the amount invested with the total cost of purchasing or establishing the enterprise. For a Portugal Cove-St. Philip's electrician buying an existing five-employee contractor, the relevant cost is the purchase price plus the working capital needed to run it, and an investor who pays that in full has invested one hundred percent.
A rumoured minimum figure is not the rule and can mislead in both directions: a smaller full-price purchase of a genuine operating business may qualify, while a larger sum that represents a small fraction of a costly enterprise may not. Separate from the investment are government charges, which for E-2 include the visa application fee and any reciprocity fee set by the Department of State, and any USCIS fees if status is sought from inside the United States, all published on the official fee pages. Professional fees, state licensing, insurance and bonding are business costs, not immigration costs, and they should not be counted as part of the investment unless actually spent on the enterprise.