Learning Facebook advertising is a science in itself, fFucking complicated as fuck and i am thinking to hire a marketing agency dedicated to handle this fking bullshit for me, its about mobile android/ios games, what do u guys think?..... How much money would that require to pay them?

How many hours daily do you think it will take them monthly? Add 8 to that number (a day to talk to you, evaluate the proposal and allocate a resource).
Now multiply it by 30 again (salary of manager + salary of resource + taxes + monetary gain).
This is a conservative estimate.

First you have to know where and to who you want to advertise.

Let's say you pay $10 for one day of fb ads which gives you 5000 views. 10% click on it and an optimistic 4% (200) buy your game for $1 after seeing the advertisement. $200 - $10 = $190 in revenue a day. Now how much marketing budget do you need to be profitable to offset the cost of doing business and development cost to be profitable?

@M1sf3t you're right but it still helps to know how the formula looks like, just so you realize that 500 dollars is lower than what they're going to ask you.