Hello everyone!! Need help in this situation!

Hello everyone!! I started collect DB tcg like year ago cause want make a super picture of all the history of DB from the start to the actual point with the all arts of this cool cards. I asked like 1 hour ago in a local store that sell Warhammer, Pokemon tcg, miniatures... And have this one;

Manga Booster 02 box sealed (400€) WTF its this price!!!

(I'm from Spain, the reason i put €)

So my question is... Should buy It? Its crazy price!! Have nice cards but... Are really really really cool for the price? Don't know if i should buy It and sell for buy the ones that i want and like. Don't know what i should do

Any advice?? Thanks!!! Everyone!! :)