At the moment you only know an extension is commercial if the description explicitly says so (which takes time depending on how long it is) or if a company posted it in the first place (most likely will include a link to the purchase site in profile). It would be great if there was some kind of indication of this even while just searching for plugins, instead of having to click several times to see the same information. Many users don’t want to spend money on extensions, and the reverse is true too: many people search with the intent being to get a quality, commercial extension.
Perhaps you could write a simple filter for search results. Or simply add little tags/descriptors for each extension. It shouldn’t be that difficult to implement, and would be a ton of help.