Calculations coming out wrong

I am working on making a car calculator for loans, I have:

#!/usr/bin/perl
print "Content-type:text/html \n\n";
use CGI qw(:standard);

my $cost       =    param('cost')     || '0.00';
my $payment    =    param('payment')  || '0.00';
my $interest   =    param('interest') || '0.10';
my $down       =    param('down')     || ($cost * 0.20);
my $loan       =    param('loan')     || '5.00';

my $due        =    ($cost - $down) * (1 + $loan * $interest / 100);

my $month      =    $due / (12 * $loan);

# if nothing default values below

# print output of all entered values

my $output = <<_END_;

<b>Cost:</b> $cost <br />
<b>Payment:</b> $payment <br />
<b>Interest:</b> $interest <br />
<b>Down:</b> $down <br />
<b>Loan:</b> $loan <br />
<br /><br />
<b>Amount due each month:</b> $month

_END_
print $output;

And I am getting the wrong output for the monthly amount due. Any suggestions on what I can do to fix the output, it is coming out very large.

Link: http://elan.champlain.edu/~rcoughlin32001/carcalculator.html

Thanks,

Ryan