If there is a beneficiary who is interested in buying the car, they already own a percentage of it, so that percentage gets discounted off the open market price. They get no additional discount because they are "family".
Do a quick survey of the market for make/model/year/spec/mileage/condition, dismiss the highest and lowest priced cars from the survey, get the mean price of the remaining cars and that's the base price. Discount that price by the beneficiaries' ownership, e.g. 10 beneficiaries, 10% discount to the purchaser, and that valuation is deducted from the purchaser's eventual share of the estate. No money changes hands, the car-purchaser gets less when the estate is settled.