There is no fixed amount. The capital is measured proportionally against the total cost of acquiring or establishing an enterprise of that type, so a lower-cost business requires a higher proportion.
Proportionality replaces a threshold, and the denominator matters
Hypothetical example: The substantiality test compares the qualifying investment with the total cost of purchasing an established enterprise or establishing a new one of the same kind. For a farm-equipment repair and mobile-service business the denominator includes the purchase price, the service vehicles, diagnostic equipment, parts inventory, premises costs and working capital sufficient to operate. A buyer who acquires a modest business must generally commit a very high proportion of that total, approaching all of it, whereas a purchaser of a large enterprise may commit a smaller proportion.
Alongside the investment sit the application costs, which for a Canadian applying for an E-2 visa are consular rather than USCIS charges, plus professional fees for the business plan, valuation and legal review. Note also that the enterprise must not be marginal, so a purchase price low enough to make substantiality easy may make marginality hard.