From user's or customer's point of view:
- gratis + free: all current MOD plugins; easy
- gratis + proprietary: aka. Freeware is also easy just needs a flag for purists and free-software ideologists
- commercial + proprietary: Those should have a demo, so that one can evaluate them before purchase
- commercial + free: This is where it gets tricky:
The latter could be demo-limited: purchase is required, or it could be a full-version with optional donation.
This depends on the motivation of why the plugin-developer opted for free-software but also on marketing and is furthermore related to the "user-experience" of the MOD. Subdividing the choices even more will likely become a mess.
A GPL buyout (like ArtyFX) is IMHO not really an option. That's like hoping 1% of the rich buys it for the 99%. Also the source is not initially free and a lot of free-software devs publish the source so that others can inspect (learn) or to involve the community (get help, re-use code from others, translations, testing,...). Marketing wise a buyout makes most sense if the money is invested further (compare e.g. Blender creating the foundation -- million $) and not for paying for past work (couple of hundred $).
While optional donations are appealing, most users have little incentive to donate and those who are serious about supporting a project can donate to projects (which accept donations) already.
If the MOD is serious about building sustained community of plugin-developers, selling free/libre plugins just like proprietary ones makes most sense (demo version, purchase for full version).
Now, the best of both worlds would be a "pay as much as you want, but at least X" (compare to Ardour's "price of a meal in your country of residence"). A busker can get away with the set price while a Starbucks-addict professional musician can throw in a couple of scones Also with a minimum price is $0, that'd be like a donation.
Do you think this is a viable solution for the MOD?
Users who cannot afford a purchase, can learn how to compile from source or ask someone to do that for them.
PS. One possibility that hasn't been included are "subscriptions" or elaborate marketing schemas "subscribers get a discount on purchases" or "gratis updates while subscription lasts". That can not only keep a marketing dept busy but also has technical issues "need to be online to verify subscriber status" etc.
What do you think?