A first try

my $width = 30;
my $length = 50;
my $radius = 10;
my $rate = 1000;

my $cost = ($rate/10000) *
   (0.3048*$width*0.3048*$length
    - 3.14159 * ($radius * 0.0254) **2);

print "Cost is: $cost\n";