This is a question that I'm sure we've all come to at one time or another, whether it's about game development or making a comic book or playing in a band, or any kind of creative pursuit that isn't your day job and takes away from what can be described as "free time."
My answer always depended on how much fun I was having. I started with the free C2 license back in Q4 2012 and thanks to the amount of resources available here and on YouTube, I found out 2 things:
1) It was a lot easier than I thought to make a video game.
2) I wanted to take these skills I had learned and make the best video game I thought I could make.
In order to accomplish #2, I decided I needed to drop the $130 on a C2 license. I haven't made any wildly popular games, but I have a lot of fun making them and I like working on a canvas that I know is as limitless as my skill set.
I guess the answer for you here kind of depends on whether or not the success of what you're making impacts the value of the time/money you put in. Not an unfair question to ask. So I guess the equation looks like this:
$$ for C2 license + time invested to learn the engine + gaining new skills + lots of fun + commercial success = Worthwile
$$ for C2 license + time invested to learn the engine + gaining new skills + lots of fun - commercial success = Waste of time
Mileage varies on the time it'll take you to learn and the amount of fun you'll have. But it seems like the deciding factor in this case is the commercial success. Which I don't think is an inherently bad thing to want. Like others said, you'll just have to spend some time really pumping the game up and getting the name out there, and that's work you'd have to do regardless of the amount of $$ for the license or the time invested to learn and build the game. Probably as much/more time will be spent marketing as the time you spent building the game if you're a one-person show.
I'd say if you have the free version and you know you've done all you can with it (which is a lot) and you still aren't sold, then going premium probably won't suddenly change your mind and suddenly make it all feel worthwhile.