Paying £100 for a months worth of play in a very limited dogfighting scenario - hmmm, is it worth it? Only you can say whether its worth it for you.
But personally I would say even a fantastic full game is not anywhere near worth £100 never mind paying that amount for only a months worth of play. And we dont even know what this game is going to play like yet. It could be rubbish. Just be patient and wait for the full game to come out. There is no need to even buy the Beta, you will regret it when you get fed up with playing the game before its even come out. I think most Alpha backers will be that that point already. They have wasted their first play of the game on an unfinished dogfighting game. The hype is great here but dont get sucked into it. Go play a few other games for six months and come back then.
I do not think that I have wasted anything. I have enjoyed each Alpha phase that was released. And I am expecting Alpha 4 and other future updates and releases even more enjoyable as they will introduce nonlinear gameplay elements.