There is no fixed threshold. Substantiality is judged in proportion to the total cost of buying an established business of this kind, and it must be enough to make successful operation likely. Buying an entire farm ordinarily produces a strong ratio; buying a small share of one usually does not.
The number is a relationship, so show both sides of it
Because no minimum exists, a file that simply states an amount has proved nothing. Renaud should present the total acquisition cost, the amount he has irrevocably committed, and the relationship between them, then support each figure. The allocation across leases, permits, stock and gear does that work when the valuations behind it were prepared commercially.
Beyond the investment itself sit costs that a purchase of this kind reliably generates and that should be budgeted separately: a business lawyer for the agreement and the lease assignments, a regulatory adviser for the permit consents, an accountant to document source of funds, escrow charges, a physical count of stock in the water near closing, and the working capital a farm consumes before its first harvest under new ownership. Government charges are the small figure here, published for consular applications and in the USCIS fee schedule at Form G-1055 for filings made in the United States.