There is no set amount. Substantiality is judged against the cost of acquiring this particular business, so a modest purchase price demands a proportionally higher share of it to be invested.
Proportion, not a threshold
Hypothetical example: The comparison is between what has been invested and what it costs to buy an established enterprise of this type, or to create one from nothing. A very expensive business can qualify with a smaller proportion committed, while an inexpensive one effectively requires the investor to have funded nearly all of it. That inverted relationship surprises people who assume a bigger deal is harder. For a resort purchase the total cost includes the price, working capital, equipment, licensing and the funds needed to operate through a first off-season. Presenting the total honestly, and showing what portion has actually been committed, is more useful than arguing about a number that has no fixed value in the first place.
List the acquisition price, start-up spending, working capital and financing terms before deciding whether the proposed amount is substantial. Hypothetical example: An artisan-soap maker pays a modest price for an existing storefront but must also buy inventory, insurance and production equipment. The decision is whether to commit those necessary funds before applying, not whether to leave them in a personal account as a future possibility. A credible total-cost schedule makes that choice visible.
Review the schedule with the seller and accountant before signing. It should distinguish business spending from personal reserves and show when each payment becomes nonrefundable.