We'll have both discount (standard) and commercial licenses, and cheaper versions of both when we release early-adopter licenses. I'm hoping we'll have early adopter licenses on sale within a few months, so if you're little brave and want to make something commercial with beta software, there'll be a license for it. (There'll also be a free edition!)
There won't be any limitations on plugins. In fact, the same goes for exporters - you can extend Construct 2 in any way you see fit, and we won't stop you. I'm toying with the idea of releasing the HTML5 exporter, the Javascript runtime and all plugins under a BSD open-source license. I can't see any reason not to, but for the time being I'm playing it safe and keeping the exporter closed while we work out the license specifics and finish core features of the editor.