With 800 euros, unless you want a crappy freelanced game, you have a couple of options:
The best surefire way of doing it is to do it yourself, as Kerrigan Fairbairn said in his answer, because those 800 euros won't get you much more than a laptop, Unity, and freelanced artwork. The game could be a hobby of yours that you spend multiple years developing, heck, open source it and random people might help develop it! It be an amazing learning and developing experience that you'd be sure to enjoy after you lasso the basics.
If you truly believe that this is a great game idea and you do not have the time or expertise to spend 6 years working on it, your best bet will be Kickstarter. Use the 800 euros to get some fancy drawings and animations, then have a couple freelancers make you a great Kickstarter video. Using the remaining cash, rent out a studio for a night for the video to demonstrate the low risk. If you're successful, you have just converted those 800 euros into 350,000 USD. Now partner with a moderately sized game company depending on the scale of the game and, make a game!
Or, you could just wait till the AI uprising occurs and by a robot to make your game. :)