it is just the price when other apha are free like you buy the game and get accces or even discount on the game and my famly think it is better to get the game after the bug are work out so you can just have fun
The alpha comes with a few things you have to understand
1) There are bugs, there are grievances, it's unfinished. However i find this is all part of the fun, watching the development process and getting excited about updates. Planet coaster is the most polished alpha i've seen i think, the game is great fun as it is set to only get better.
2) You aren't buying a finished game at all. There is a limited number of objects, rides and coasters. You aren't going to be able to built everything you want right away. You must be patient, finished game is still quite a way off. If you only want to play a nicely polished, finished and full game, it's perhaps worth considering waiting as PC is not that. It is however possible to have many hours fun on PC building and riding the rides.
3) The game is expensive, it appeals to enthusiasts and the theme park community is a strong one who are seemingly willing to pay the expensive rates. I know i am, just could no resist. You will save money by waiting, so ask yourself if you want to pay or you want to wait.
i would strongly strongly recommend anyone buy the game, i love it. You do have to understand what you are buying, though.