In the Mobirise official website you can find multiple extra themes and plugins that can improve the way your website looks and feels.

The most important is definitetely the Code editor, and the reason is simple, as soon as you'll want to customize your mobirise website, you will realize how important an HTML code editor is when building a website.
A website builder without a code editor is very limited, in fact this plugin allows you to make some extra modifications that you wouldn't be able to make without.

So if Mobirise is actually free, How expensive are plugins and themes? Very cheap!
Clicking on this link you can see all the plugins and themes available for sale.
There are many specials offers during the year, for example on black friday, you might be able to catch some special deal. Most plugins and themes are available for less than 50$ each.