Array
(
[Dima] => Array
(
[0] => Array
(
[id] => 61
[price] => 9
[time] => 2018-03-13 18:31:01
)
[1] => Array
(
[id] => 62
[price] => 6
[time] => 2018-03-13 18:31:01
)
[2] => Array
(
[id] => 63
[price] => 32
[time] => 2018-03-13 18:31:01
)
)
[Nick] => Array
(
[0] => Array
(
[id] => 65
[price] => 68
[time] => 2018-03-13 18:32:01
)
[1] => Array
(
[id] => 67
[price] => 91
[time] => 2018-03-14 00:00:02
)
)
)Array
(
[Dima] => Array ([price] => 32) //максимальное значение прайса у Димы
[Nick] => Array ([price] => 91)//максимальное значение прайса у Ника
)
$new_arr = [];
foreach ($arr as $name => $item)
$new_arr[$name] = ['price' => max(array_column(array_values($item), 'price'))];