I am a self employed minibus owner driver in Manchester uk .A
transfer cost for 4 people normally incorporates a saloon car ,above 4 and up to 8 would usually be a minibus .Typically a minibus would be half as much again as a saloon car ,eg £20 for 4 persons in a car £30 for 6 persons in a minibus.
An added problem can be that when 4 people travel with luggage a saloon car is unable to accomodate all the luggage so 4 people with cases may indeed need a minibus and have to incur the extra cost.