$arr = array(
    'Ford'=> array(
        array('id'=>1,'name'=>'Alex','cost'=>12.50),
        array('id'=>2,'name'=>'Max','cost'=>55.11),
        array('id'=>3,'name'=>'Igor','cost'=>18.00)
    ),
    'BMW'=> array(
        array('id'=>1,'name'=>'Dmitry','cost'=>16.30),
        array('id'=>2,'name'=>'Vlad','cost'=>15.55),
        array('id'=>3,'name'=>'Stas','cost'=>8.00)
    ),
);

['BMW']['Price'] = (16.30 + 15.55 + 8.00) = 39,85

foreach ($arr as $key=>$val) {
	$sum = 0;
	foreach ($val as $m) {
		$sum+=$m['cost'];
	}
	$arr[$key]['Price'] = $sum;
}