There's no reason why this can't be made public knowledge before the event.
Not necessarily a full penny by penny breakdown, but I would expect all advertising for the event to have a minimum of "Cash prizes for the top x amount of players" or something similar.
Then on it's website, or FB page or similar a more detailed breakdown of the payout such as
It should also be clear what money is being taken from the prize pool, whether that be for venue hire, trophies, fuel payments to donors, advertising, food for volunteers, charitable donations, organisers payment for time, etc.
If for no other reason than to protect the organiser from accusations of lining their own pockets.
There isn't really this issue in the UK, as the vast majority of tournaments - even the larger ones - don't award prize money. The way the entry fees are allocated in the tournaments I run is usually along the lines of: money taken out for trophies, the remainder is split 50% to the host, for his time and effort in hosting with the other 50% being donated to the charity of my choosing. I don't a big deal about me donating to charity, but anyone who asks (and I've been asked once in the last 3 years) will be told and can see the receipt for the donation.