No fixed minimum is published. Substantiality is measured proportionally against the total cost of buying an established business or creating a viable new one, so a lower-cost enterprise requires a proportionately higher share of that cost. Government fees are separate and are set out in the official fee information for the relevant filing route.
Proportion decides it, and small businesses are held to a higher percentage
The absence of a stated figure is the source of most confusion, and the proportional test explains it. The comparison is between the amount invested and the cost of the enterprise in question, so an investment representing a large fraction of what the business actually costs is more persuasive than a larger absolute sum representing a small fraction of an expensive one. A calibration laboratory whose value sits largely in accreditation, technical staff and reference standards rather than in real property will therefore be expected to be very substantially funded.
Costs beyond the investment itself deserve their own line: valuation, legal work on the purchase, accounting review of the seller's figures, and the filing charges applicable to the route chosen. Because those charges are revised periodically, current official fee information should be consulted rather than a figure quoted second hand.