<?php

$string = '1-350,9-390.99';

$output = array();
foreach ( explode( ',', $string ) as $pair ) {
    list( $key, $val) = explode( '-', $pair, 2);
    $output[$key] = $val;
}

var_export($output);

array (
  1 => '350',
  9 => '390.99',
)