Signalling server is free. You can buy your own, but scirra provides it for free.
If you buy it, then you need to host it and have SSL etc etc etc.
Scirra has a state of the art server and can handle anything you through at it, and it is free to use.
This would be your cheapest option really. And If you hit new in C2 and search multiplayer you have a few examples right there. And in the tutorial section ashley and a few others have put together some in depth tutorials.
If you still can't grasp it after that, and lots of banging your head against the keyboard, then it might be too advanced for you.
There is also a cool multiplayer tutorial using mysql and php and of course as you've stated you can use socket.IO, but seriously, I would invest my time using the multiplayer functionality of C2.