the price you pay now includes the game at release so you wont be charged extra.
As to whether its worth the money now I honestly cant recommend it in its current state, your effectively paying for something which until recently was free (alpha/beta access)
The developer has no obligation to finish the game in that sense, they already have prob 200k sales which will fund the game, but if dean took off tomorrow with a suitcase full of money we would all be left holding onto our collective nuts.
Saying that im enjoying the game, its very very bugged and if your playing it as a loner you are better having the scavenger mentality than hitting up the cities.
As long as Rocket delivers on all his promises then it will eventually be amazing, at the moment its good, but only if your prepared to put up with its many faults.
If your undecided and have Arma2 and arrowhead then give the Mod a go, see if you like that before taking a plunge.