There is no minimum amount set by the category. The investment must be substantial in proportion to the total cost of buying or establishing the particular business. Government fees depend on the route taken, and USCIS filing fees appear in the fee schedule, Form G-1055.
Proportion decides substantiality, never a headline number
Two purchases of identical price can be judged differently because the cost of the enterprise differs, and a figure someone quotes as the going rate for E-2 has no legal basis. The proportion Fatima has committed relative to what the chip-hauling company actually costs is what matters, and the closer that proportion is to the whole cost, the stronger it reads. Never import the EB-5 rules here: there is no ten-job requirement and no statutory capital threshold in E-2.
Beyond the investment itself, budget for legal work on the purchase agreement and the note, accounting review of the seller's records, valuation where the price is negotiated, and the application costs applicable to the route chosen. Confirm any USCIS fee on Form G-1055 at the time of filing.