The folks over at YOAST! posted a great article on why a quality plugin sometimes is worth the spend.
While many paid plugins are just hyped versions of freely available work to those who know how to look, sometimes, a Developer sells a plugin so they can devote time to quality.
We use and have used YOAST! SEO tools for a good time now in our WordPress installations.
The latest blog post from them shows why, good Developers can explain their actions, and are worth their price.
Full read over at YOAST! follows: