No figure is published, because substantiality is proportional rather than absolute. The amount is weighed against the total cost of buying the business, and a lower total cost demands a higher percentage. Separately, the consular or filing charges are small and are published in the official fee sources.
Price the whole undertaking, then test the proportion
The mistake is to treat the purchase price as the denominator. A realistic total for Wendell includes the price of the shares or assets, the deposit and closing costs, the first season's payroll before revenue arrives, insurance and bonding, the dockage or yard lease, licensing, and the replacement of equipment the seller has run to the end of its life. Once that total exists, the question is what proportion of it he is actually putting in and whether the amount is of a magnitude that supports the likelihood of successful operation.
A purchase funded to the last dollar with nothing left to run the business argues against him. Administrative charges are a separate and much smaller matter: an application made abroad through a consular post carries the visa application fee, while a change of status filed inside the United States carries the fee published in the USCIS fee schedule at Form G-1055. Professional fees for counsel, accounting and the business valuation usually exceed both.