You can ignore all the extra features and focus on landing pages, which is where Premise is really good. (I find the membership and checkout features to be a little iffy.) There are a ton of included graphics, style settings, and different ways to layout the pages, though you won't get the large variety of templates that Unbounce has. Premise also comes with a large library of videos and seminars on copywriting, SEO, conversion optimization, etc. You can also integrate it easily with Visual Website Optimizer, and hopefully soon with Google's new Content Experiments.
Overall I find it a pretty good alternative to Unbounce.
Thinking about it now. It doesn't really matter if it's a plugin since I can just install Wordpress in a subfolder.
Does the URL matter for a landing page? For example, if I have product A's URL as widgets.com/products/a. Would it be fine to have the URL of the landing page for this product to be: widgets.com/landingpage/a. In this case, I would've installed Wordpress+Premise in /landingpage.
It sounds like you need to create landing pages repeatedly, and you don't want to pay recurring fees, so why not get some landing page templates from ThemeForest or somewhere, and use it on your websites with your styling? I guess it'd take some work, but you could make them customizable for your needs.
I want to optimize landing pages by continuously performing A/B tests through Visual Website Optimizer, which already allows easy editing of the web design.
I'll look into the themes from Themeforest but I just thought there'd be an easier way (DIY perhaps) to create an appropriately themed landing page.
Thanks again!