Substantiality is proportional: the investment is compared with the cost of buying or establishing the specific business. There is no fixed minimum. Application costs include the consular visa fee or the USCIS change-of-status fee, plus professional and diligence costs.
Measure the investment against the business, not against a rumour
The Department of State applies a proportionality test: how much of the total cost of purchasing or establishing the enterprise has the investor actually committed? For an inexpensive service business, a high proportion is expected; for a capital-heavy operation, a smaller proportion of a large figure can be substantial. The money must also be enough to make the business likely to succeed.
Hypothetical example: a Boischatel contractor paying most of the purchase price of a landscaping company in cash, with the balance secured on his Québec home rather than on the business, shows both substantiality and personal risk. Government charges are the visa application fee at a consulate, or the I-129 filing fee if changing status inside the United States, on the USCIS fee schedule. Around those sit the real costs: accounting and legal diligence on the target, a business plan, translations, and the working capital the plan promises, which the consulate will expect to see funded.
A dependant's application carries its own fee.