Hello, LunaBunPlays. Thank you for playing :)
The demo is only the very first part of the game. I do not want to throw inaccurate percentages, but I would say the demo is about 10% of the complete game. Of course, I play it differently from you, since I always know what to do and where to go, so take it with a grain of salt.
Also, if you are not sure because of the price, follow the game page and wait for a sale: Psycologico was highly discounted during April, so it may happen again in the near future ;)
Cheers!