You and they have to voluntarily work it out--that is, mutually agreed on something. If there is no buy-out agreement, which there evidently isn't, you have no right to buy out a minority shareholder and they have no obligation to sell to you, and no formula, etc. to determine price--if they choose to sell and you choose to buy, it's because you each want to do so, which means you have both agreed on what will be paid. Each can ask for or offer whatever they want. He can ask for the price for the share plus the cash and refuse to sell unless you give him that; you in turn can refused to pay that much and so refuse to sell. Ultimately, as stated, you and he have to work this out between the two of you; the law does NOT fix a price.