diffrence between extra and comm is mainly multiplayer support and shader support. And some other restrictions. Imo it's pretty much reasonable for that amount of money and worths $100.

If you are going to go on with GS, even if you buy extra version some time later(after you get hang of it with extra) you'll want to use shaders,then you'll upgrade to comm which will cause you a $50 loss.

Trial represents PRO afaik.

(For multiplayer part you can use 3rd party mp plugins which even works with free version.)