There is no fixed minimum; the investment must be substantial relative to the cost of acquiring her share and the planned operation, and sufficient to make the venture likely to succeed. Government charges are the consular visa fee, or the I-129 fee for a change of status, plus professional and diligence costs.
Substantial is a ratio, and the ratio is set by this business
The Department of State's proportionality test compares the funds committed with the total cost of purchasing or establishing the enterprise. For a service business bought at a modest price, the investor is expected to have committed most of that cost from her own funds; for a larger, capital-intensive purchase, a lower proportion can still be substantial. The funds must also be enough to operate the business as planned, which the business plan should show.
Hypothetical example: a Shannon contractor buying half of a Virginia fire-protection servicing company and funding new service vehicles from her own retained earnings has committed a high proportion of her share's cost, which supports substantiality. Application charges are the consular visa fee, or the Form I-129 fee on the USCIS schedule if she changes status from inside the United States, plus fees for each dependant. Legal and accounting diligence on the partner's business, the operating agreement drafting, translations and the business plan are separate professional costs that protect both the investment and the visa.