There is no statutory minimum; the test is whether the investor's own committed funds are substantial in proportion to the cost of purchasing or establishing the enterprise and sufficient to ensure the investor's commitment to its success. For an established business, the relevant comparison is the investor's at-risk contribution against the total acquisition cost.
Run the proportionality test on your own money
Compute the total cost of the acquisition: purchase price, working capital needed at closing, equipment refurbishment and licensing costs. Then compute the amount that will be the investors' own capital at risk after excluding any financing secured by the business. The ratio between them is what the consular officer weighs, and the closer the business is to a low-cost enterprise the higher the proportion expected.
A buyer who contributes a modest sum and finances the rest through the seller against the company's assets has a low ratio and a weak case; the same buyer who draws on a home-equity line and a personally guaranteed loan has a high ratio. Government charges are separate: the visa application fee, any reciprocity fee, and later, if status is extended inside the United States, the USCIS fee in Form G-1055. Budget also for the state licensing process, since a company that cannot lawfully bid on work is not operating.