I would first calculate the daily/monthly rate for the month that falls in a particular time period, then find out how many users logged on during that month, then distribute the cost evenly to the users. The following is a sample I quickly pulled together to show how to calculate daily rate. To calculate the monthly rate, say, you just need to accumulate the totals for the month.

use strict;
use Date::Calc qw(:all);
use Data::Dumper;

my $rates = [
    ['01/01/1990', '09/01/2003', 105],
    ['08/01/2003', '10/01/2003', 190],
    ['10/01/2003', '01/01/2007', 140],
];

my $new_rates = initrates($rates);
# print Dumper($new_rates);

my %date = ( '07/01/2003', 20,  # date, no. user
             '09/01/2003', 10,
             '10/01/2003', 20,
             '12/12/2003', 12);

foreach (sort keys %date) {
    my $rate = getdailyrate($_);
    my $per_user = sprintf "%.2f", $rate / $date{$_};
    print "Daily rate for $_ is $rate, $date{$_} users, every user pay
+ \$$per_user\n";
}

# date in dd/mm/yy
sub getdailyrate
{
    my $datestr = shift;
    my ($y, $m, $d) = getymd($datestr);
    my $date = Mktime($y, $m, $d, 0, 0, 0);

    foreach(@$new_rates) {
        return $_->[3] if $date >= $_->[0] && $date < $_->[1];
    }

    return(0);
}

sub initrates
{
  my $rates = shift;
  my @rates;
  foreach (@$rates) {
    my ($y1, $m1, $d1) = getymd($_->[0]);
    my ($y2, $m2, $d2) = getymd($_->[1]);
    my $total = $_->[2];

    my $dd = Delta_Days($y1,$m1,$d1,$y2,$m2,$d2);
    my $rate = sprintf "%.02f", $total / $dd;

    # print "$dd days - wanted $total - rate \$$rate per day\n";

    my $t1 = Mktime($y1, $m1, $d1, 0, 0, 0);
    my $t2 = Mktime($y2, $m2, $d2, 0, 0, 0);
    push @rates, [ $t1, $t2, $total, $rate ];
  }
  return \@rates;
}

sub getymd
{
    my $datestr = shift;
    my ($day, $mon, $year) = $datestr =~ m[(\d+)/(\d+)/(\d+)]; # or Pa
+rse_Date
    return ($year, $mon, $day);
}
[download]

And the output is -

Daily rate for 07/01/2003 is 0.02, 20 users, every user pay $0.00
Daily rate for 09/01/2003 is 95.00, 10 users, every user pay $9.50
Daily rate for 10/01/2003 is 0.10, 20 users, every user pay $0.01
Daily rate for 12/12/2003 is 0.10, 12 users, every user pay $0.01
[download]

Note that I have $0.00 in the output, that's due to the rounding with sprintf (for printing only). Remove the sprintf and you will get more accurate daily per person payment amount.