C2 and all its future releases/updates are a one-time fee (pay one time).
About C3, well only future will tell. C2 is still in beta, there are plans for other exporters and a lot more features to be implemented.
It is likely that it will be a new product that you will have to buy. Maybe there will be a discount, or not. Anyway we're talking about something totaly hypothetical, C2 has to be fully finished before thinking about a "next version".
You can check out the articles in th blog of Scirra to find more about some of the plans for the future.
For the present start here and there to begin your journey with C2.
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