<?php
$bills = array(
		100 => 23,
		500 => 5,
		1000 => 0,
		5000 => 200
		);
$reverse = array_flip($bills);
$billsSum = array(
				100 => $bills[100]*$reverse[23],
				500 => $bills[500]*$reverse[5],
				1000 => $bills[1000]*$reverse[0],
				5000 => $bills[5000]*$reverse[200]		
		);
print_r($billsSum);