PHP 5.5+ two-line solution, using array_chunk and array_column:
$input = '1-350,9-390.99';
$chunks = array_chunk(preg_split('/(-|,)/', $input), 2);
$result = array_combine(array_column($chunks, 0), array_column($chunks, 1));
print_r($result);
Yields:
Array
(
[1] => 350
[9] => 390.99
)
See it online at 3v4l.org.