The price for his half, held in escrow on visa-conditioned terms, plus any working capital or equipment he commits to the firm. Personal reserves, a house, and any deposit he can recover at will do not count.
Count what is at risk in the firm, and only that
The investment is the amount the buyer has placed at risk in the enterprise. The purchase price in escrow counts once the escrow is irrevocable except for visa refusal. Funds he contributes to the firm's bank account for working capital, and equipment he buys and titles to the firm, count as well.
Money kept in his own account as a cushion, a residence bought for the family, and a deposit the partner would return on request do not. The government charges are separate: the consular E-2 application fee, or the Form I-129 fee in the USCIS fee schedule (G-1055) if he changes status from inside the United States. Legal fees for the purchase and the application are private costs.
Present the numbers as a dated schedule so the substantiality comparison, investment against total cost of the half-share, can be checked line by line.