<?php

$list = array (
	1	=> '215070121',
	5	=> '215001216',
	12	=> '215050121',
	21	=> '215010127',
	50	=> '215080122'
);

function getClosest($search, $arr) {
   $closest = null;
   foreach ($arr as $price => $id) {
      if ($closest === null || abs($search - $closest) > abs($price - $search)) {
         $closest = $price;
      }
   }
   return $arr[$closest];
}

echo getClosest(10, $list);