The free version is enough for your first month of discovering C2, at least. You won't have that time though if you plan on the steam sales.
You can do whatever you want on C2, but it's optimized for games, not apps, meaning it will be more intuitive to make a game than a calculator. It's won't be hard or impossible to go for a calculator though, just less intuitive.
And I'm all with California's first remark : any prototype is about 1 hour work in C2, where it could be 2 to 3 days in jQuery, PHP, ...