I own a small web design company and would like to being offering my
clients a web based email newsletter service.  In the past I have
simply turned them to pre-built services like constant contact, but I
would like to begin being able to offer this as an add on to their
site, and use my company logo to brand it.  I have an immediate need
to install a mailing list application on 8 different domains, on
different servers, but all
running PHP / MySQL, and would like to be able to license it to up to
appox. 30 sites a year.  Here is the functionality I am looking for:
- The ability for site visitors to be able to register with their name
and email (more fields would be nice, but not necessary) on the front
end.
- A web based list management interface giving the admin the ability
to add/delete/edit names and emails.
- Send HTML emails to all or some of the members of the list (Groups
would be good, but not nessasary).
- Email lists would be no more than 10,000 names, and in most cases
much smaller (less than 500)
- Click through and other usage statistics
- Store upto 3 HTML templates and allow a non technical person to
select which template they want to use and fill in the text and have
all the formatting done for them.

This is something I would like to be able to install on multiple sites
on different servers, and continue to install on new sites I develop,
so a one time fee to buy a package that can then be used on unlimited
sits would be best.

I have only done preliminary research at this point, and have found
scripts that require licensing per domain (no good), or are offline
solutions (needs to be web based), or use ASP (needs to be PHP), or do
not allow branding (I would like to be able to have
clients see my logo each time they send emails), or do not sent HTML
emials, or do not allow templates (this could be compromised on, but
having them would certainly be much better), or are $10,000 to
license how I have described above (much more than I?d like to spend,
under $1,500 would be preferable).

I would like suggestions on which are the top web based email list
managers would work best for my situation, as well as price,
difficulty to install and configure, and features and benefits of
each.

Thanks in advance!

Why dont you get it built by some one? It should not take more than a
month to build this kind of a system.

Thanks for your suggestion!  That would be a good option if there is
not one a pre-built system already developed that meets my criteria. 
I believe there are many people in a similar situation, and would
think there has to be one tailored to what I need already.  If not, I
will explore this option - how much do you think something like that
would cost to have programmed?