[5] =>  Object1
    (
        [id] => 1
        [name] => abb
        [costs] => 33
    )

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

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

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

function mySort($a,$b) {

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

'id' => 'service5',
 'name' => 'YouTube',
 'costs' => 8064,