Enough that the investment is substantial relative to the cost of opening this specific roastery, which for a modest startup usually means most of it. A reserve for living expenses is sensible and should be identified as such, not counted as investment.
Substantiality is a ratio, not a threshold
The rule has no dollar floor. It asks whether the amount invested is substantial in relation to the total cost of purchasing or establishing the enterprise, and it applies a sliding scale under which a lower-cost business needs a higher proportion of investor funding. A roastery and café might be established for a sum the store proceeds would cover in full, and the review will expect the couple to have committed most of that sum rather than a token fraction.
Working capital that has been transferred to the enterprise account and is needed for the first months of operation can form part of the investment if the plan supports it. Money held back for the family's rent and living costs is not investment and should not be described as if it were; it is evidence of prudence. Consular application fees and reciprocity charges are set by the Department of State; consultation fees sit outside them.
Do not spend on immigration services before the commercial numbers are settled.