There is no fixed minimum. The investment must be substantial in relation to the total cost of purchasing or establishing the particular enterprise, and sufficient to ensure the investor is committed to running it. A modest facility therefore needs proportionally less than a capital-intensive one, and proportion is what is measured.
Proportion, commitment and risk rather than a number
Hypothetical example: Colette is deciding how much to commit to an American self-storage purchase. The absence of a stated threshold surprises people who arrive expecting one. Substantiality is assessed relative to the cost of the business being bought or built, so the relevant arithmetic is the ratio between what Colette commits and what the enterprise costs in total, together with whether the amount is enough to make it likely she will successfully operate it.
Funds must be irrevocably committed and at risk, which means money sitting in a personal account is not an investment and an escrow that returns everything on refusal must be drafted with care. Around the purchase price sit the real costs of the move: legal and accounting work on both sides, due diligence, any franchise or licensing charges, working capital, and the fees shown in the current USCIS schedule where a change of status is filed rather than a visa application made abroad. Working capital the business genuinely needs forms part of the investment; money held back in case the application fails does not.