If you really like it, I suggest getting it. I love the game myself. If you can afford it, get the Alpha version too.
Pre-ordering the game and getting early bird access to the Alpha version on Frontier's site they charge like 49 pounds which is equivalent to approximately NZ$103. Even some Xbox One games sell for around $119 in NZ.
As for the map size, I think the size Frontier sets in the Alpha version is big enough. If you run out of room, you can always make a new map and build some other creative stuff on it.
I think it may eventually come to Steam after the full game releases. Steam might charge slightly less for the game.
The game is expected to come out Q4 2016 = October/November or December this year (unless Frontier changes it).
If you have a job try putting money to it or work a few extra hours once to get the money for it. If you're younger, try babysitting or walking a neighbours/friends dog or car washing (with some friends) so you can get the money for the game.
Just to let you know, you'll need a good PC to run it too. I have a white HP 15' laptop with Intel Core i5-6200U (2.3-2.8Ghz) CPU, 8GB RAM, and AMD R7 M360 2GB GPU, and it struggles to play this game at 1080p Low settings. So this week I'll be receiving a gaming desktop PC which I ordered through a special NZ company who lets you customize what you want in it and they build it for you and courier it to you. The system I ordered has an Intel Core i7-6700k with Sapphire Nitro AMD Radeon R9 390 8GB GPU.
