C3 (as I read it) is basically C2 updated.. it's not some radical departure from how C2 works. We have been told that Capx files will open in C3.. there may be some things that change (consider how webstorage was compared to localstorage plugin, a significant change, but necessary move)
As for plugins yep.. you can't in Unity3D either. If I ask $500 for a plugin.. or $5.. it's up to the C3 user to choose if it's worth it. a star rating system will help. Plugin developers will ask themselves if it's worth their time to make a plugin for $3.. and have to sell 300 to make what I deem worthwhile for me time... because I had to spend a week figuring out all the challenges of a given service / conversion. Are there 300 C2/3 users who will even want my plugin? I don't know.. however, I'd love to make $10000 for my plugin too so i can spend more time answering users with problems (if there are any) knowing good reviews and happy customers will drive more sales..
C2 gives you the ability to make your own plugins.. C3 will likely be no different. It's basically .js files with some editing done to expose certain parameters to the editor to act on the plugin's code. Also earlier I said plugins are easy.. this is a bit oversimplified.. some are very easy.. others can be very involved.
you gotta yourself as a game developer.. would you rather pay a little more and know you're giving a developer enough value to care to support you or would you rather pay $3 for a plugin and wait weeks to hear back from them? I'd rather see developers get paid for their time... and be there for me if I need them so I can spend less time pulling my hair out because something is wrong and no one will reply. If someone charges a premium for a plugin then skips town.. nothing can stop that from happening but ratings will suffer and people will warn each other..
However I get the feeling from the interactions I've had with the C2 community that it's a good group of people and I believe that the players we've seen making fine plugins so far are going to be continuing to do their good work. Frosty Elk's Parse plugin was great and I did get email support from them when we had issues with it AND our testing found a bug which helped them improve the plugin for everyone. I paid $9 i think for that and it was a great value, but I feel they could charge more and have the time / incentive to do more with it / help people.
I've never heard any developers say how much they have made off any plugins, but I imagine the money they made was modest. how many people really are using parse with C2? We want too so we bought the plugin. I haven't tried Cranberry Parse yet, but I have it because I paid I think $35 to have full access to all of the plugins which is insane value, however that value becomes less when you realize the developer (understandably) doesn't have time to support every issue because there isn't enough money coming in to justify the time it takes to support every issue that comes along.