[6] =>  Object2
    (
        [id] => 2
        [name] => bcc
        [costs] => 55
    )

[4] =>  Object3
    (
        [id] => 2
        [name] => fff
        [costs] => 66
    )

[3] => Object4
    (
        [id] => 4
        [name] => fff
        [costs] => 55

if($a->costs > $b->costs) return -1;   - работает
if($a->costs < $b->costs) return 1;    - работает
if ($a->costs == $b->costs)  {         - не работает