How about (this would need a new tag but that's not hard) if it was a percentage of the income from the targeted object (so player income if its a player event, city income if its a city event)?
that would be very unrealistic and a bit ridiculus
when you host the olympic games and you are rich they cost you 1000
and when you are poor 10

i propose something like that
except options if you want to do something or not
have options on the same event about
how you want it to do
example
A do you want to host
a magnificent Olympic games
5000 many benefits and rev stability increase in cities
2 do you want to hold a prestigious O.G.
2000 gold les benefits but still big benefits
benefits
3 host normall O.G. . 1000
some small benefits but there is a small chance for some negative one
4 be cheap

500
many negative things +rev stability decrease
5 dont host them at all
small negative things like happiness and prestige hit)
B A stadium was colapsed
1 give large amount of money 2000
the buildings get a bonu like
-5 maintanance for 100 turns
or -5 % to collapse in the future
2 normal 1000
3 be cheap and give few
there is a very small % that the building wont be fixed
but also +5 maintanance for 100 turns
or +5 % to collapse in the future
also the event cost should scale at the case
(repairing huge stadium costs more than a shop