Hi, KiethG-
Your question is a common one as the multi-portal ability of DNN certainly seems to lend itself to easily providing a shared-hosting environment with easy management of the underlying DNN platform and its modules.  There ARE some issues with doing this (see "Downside" , below), but first, here's a module that does what you asked about: 
  
 Portal Sign-Up 5.7 by OnyakTech
 http://www.snowcovered.com/Snowcovered2/Default.aspx?tabid=242&PackageID=7360

This module allows end-users to create their own portals with an option of charging a subscription fee via PayPal.  You may set up child sites (non-unique root domain) or parent sites (unique domains, though additional tasks must be done to configure a unique domain to a new portal).  You can provide a trial period with its own fee separate from the normal subscription fee.  Finally, you have the option of allowing portal creators to be Administrator of their portal or you may disallow the Administrator role from the portal owner.
 
 
The downside....
In a multi-portal installation every portal shares the same database with the same data tables for the various modules, user lists, passwords, etc.  Any portal Administrator has access to all data in the database (i.e., all data for all portals).  Besides being a security issue, it is a risk in general since an Admin who does not know what he/she is doing could wipe out any or all of the DNN database, effectively wiping out your DNN installation.
 
With the Portal Sign-Up module you could decide to disallow portal owners from having the "Administrator" role but you'd need to provide some means to allow those users to create web pages, add modules to pages, manage portal users, etc.  Normally this is done through the Administrator role.  Further, you must take care in determining which modules you will make available for use as some modules allow direct database access (REPORTS module and UDT module are two core ones that do).
 
Due to these security concerns you'll typically see hosting providers give individual installations of DNN to individual customers rather than place customers on individual portals of the same DNN installtion.  The multi-portal aspect of DNN is often a great feature for providing multiple sites to the same customer, but it can be problematic when attempting to provide sites to different customers within the same DNN installation.
 
 
Hope that helps-
-mamlin