Oh, I have no idea. I was just speculating. This approach worked for Minecraft, so maybe it will work here too.
You pay once. There is no "monthly free", you just pay for whatever membership level you like, and that's it. lol you'd have to be stupid to pay a monthly subscription for such a thing. (oh,
wait a minute...)
Seriously though, I know this because I'm a contributor myself; a junior member to be more precise. So basically I get to play the monthly builds of the game. Anyone can do this. Just go to their forums, register, and there's a link at the top of the forums that let's you make a purchase. For 10 euros you can play it.
The only problem is that they make it extremely difficult to find where to sign up, and where to pay. They have a great product under their belts, but it's almost as if they don't
want people to use it. It actually took me a long time before I realized that you have to sign up for their forums first, and
then you actually have to pay for it in the forums. The button that says "become a payed member" or whatever is very small and well-hidden, but once you find it on the main forum page, it then lets you choose your membership level and finally you can play it.
Even their website is extremely hard to find by itself. Doing a google search for "project cars" brings up nothing, and refining it to something like "project cars racing simulator" didn't even work last time I checked. Their marketing team needs an SEO guy or something.
But yeah.... it's an impressive racing simulator, especially considering it's only in pre-alpha stages.