There is no fixed figure. Substantiality is proportional: the investment is weighed against the total cost of buying or establishing this kind of enterprise, so a modest purchase price with a high proportion committed can qualify.
Proportion decides, not a threshold
Hypothetical example: a Grand Bay-Westfield couple hear that a treaty investment needs to be a particular round number and nearly abandon a purchase priced well below it. The test is relative, not absolute. Where the total cost of acquiring an operating tree-service company is a given amount, committing nearly all of that amount is strongly substantial, and a low-cost business simply requires a higher proportion.
What weakens a file is counting things that are not investment: working capital left uncommitted, personal vehicles, or an expense estimate with no invoices behind it. Include the purchase price, transferred equipment at supported values, inventory, deposits and start-up expenditure actually incurred, and show each with a document. Government charges for the application itself are published by the Department of State and USCIS and should be checked when filing rather than estimated.