I agree that the price of the supporter is steep looking at what you get compared to more commercial products, and even a similar concept like Hattrick. However, this game is young, and has no big $$$ (EA, Valve, Microsoft) behind it and thus need our support (hence the name 'supporter') to keep developing into a game that appeals to a large community of players and gives alot of added value to its supporters.
In this early phase though I think it's important that everyone that appreciate the potential of this game, and have the ability to show their support by spending some $ on getting supporter - do it! You are investing in something that could become great fun, and I believe the BBs for sure will remember who made it happen!
In my eyes - coming over from Hattrick, and without any special interest for basketball - this game looks promising and - as mentioned above - seems to have dedicated developers behind it.
So - if you like it and can afford it - support the game!