bill.php

<?php

$bills = array (
     array (
     'min' => 1,
     'max' => 75,
     'price' => 3.8
     ),
     array (
     'min' => 76,
     'max' => 200,
     'price' => 5.14
     ),
     array (
     'min' => 201,
     'max' => 300,
     'price' => 5.36
     ),
     array (
     'min' => 301,
     'max' => 400,
     'price' => 5.63
     ),
     array (
     'min' => 401,
     'max' => 600,
     'price' => 8.7
     ),
     array (
     'min' => 601,
     'max' => 1000000,
     'price' => 9.98
     )
);

$billedUnit = 900; //User Input

$processed = $billedUnit;
$totalCost = 0;

foreach ($bills as &$bill) {
    $amount = NULL;
    if ($processed == 0) {
        break;
    }
    $maxAmount = $bill['max'] - $bill['min'] + 1;
	if ($maxAmount <= $processed) {
        $amount = $maxAmount;
	} else {
	    $amount = $processed;
	}
    
    $bill['amount'] = $amount;
    $bill['cost'] = $amount * $bill['price'];
    $totalCost += $bill['cost'];
    
    $processed -= $amount;
    
    echo "{$bill['min']}-{$bill['max']}\t\t{$bill['amount']}\t{$bill['cost']}\n";
}

echo "Total:\t$totalCost";