If you upgrade to CS6 (this will be your last chance to upgrade), there is built-in tools to help you code a template for a Wordpress site.  More than likely that will be what you are looking for because it will give your clients access to edit the site without having to purchase software like a Dreamweaver.  If you are new to web design this will be easier said than done though because at the very least you would need to have an understanding of HTML and CSS to modify a template to look the way your client wants it to look.  I will warn you though, there is a learning curve and you have to be willing to dedicate the time if this is what you really want to do.

Pricing is a difficult question.  It varies from region to region and per the requirements of the job.  Without knowning any requirements of the client it's hard to say.

The upgrade is $249 per the promotion that Adobe is running.  After 12/31/12 you will need to buy a new license if you want the new version.  It is possible that your computer might not have the specs necessary, so you should check the system requirements first since you have until the end of the year.

Comparing DW, Wordpress and Joomla are not equivalents at all. Drupal, Wordpress, Joomla, PyroCMS, Perch, etc are Content Management Systems.  They install on your web server and manage the content for you.

Dreamweaver is a web/code editor.  Meaning that you can code and create whatever you like.  It is not to manage content, but rather to create the HTML, CSS or other code.  With Dreamweaver, you could (with extensive knowledge), create Drupal, Wordpress, etc.  This is why DW built code-hinting for those platforms into DW to make editing the code of those documents easier.