Are there any good addons worth their price out for CS4/5? I was interested in the marketplace addons, as the site I am making will include a marketplace(as well as future projects)
are you talking eCommerce?
To follow Ralph’s suggestion you might look into WordPress.
I haven’t used addons for DW since version 4 when most of them were free.
Hi Mike, welcome to SitePoint! 
To be honest, I’m not sure about whether there are such addons, but I wouldn’t be surprised if there is something out there. The main reason I’m posting this answer (other than to say Hi) is to say that my misgivings about such addons is that you end up with code that only Dw understands, which is a pain if you ever want to move to another editor.
What exactly do you mean by a marketplace? It sounds like you might need some kind of content management system (CMS), which is basically PHP code that is independent of any particular editor. There are lots of free CMSs out there, too, though they will take a bit of learning.