If you buy the alpha now at £20 on Steam; I think that entitles you to the full game. If you have a dedicated interest in the game, it may be a good investment to get it now while it's cheap(ish) as the full game may retail at a typical £35-40 mark.
Problem is, once the beta is out, so is The Division so I can partly see why they are cashing now but if you do get it in alpha state; I don't think you have to pay for anything ever again related to the game.