Doesnt really matter, both sides pay equally, or better yet, they pay the organisation that employs the dayanim who are on a fixed wage.
But in this case, only 1 side is paying (well, the other side is paying, but not with cash
). So how can it be a valid sechar betaila?
If it's taken as payment for hearing the case it's Assur, even if it isn't given with the intention to curry favor for one side and the Dayan would have a status of Dinov Bateilin.
Your last question is a tricky question, the simple way is to calculate what profession the Dayan has and figure the average salary; but I'm sure there are other ways to calculate it.
Keep in mind that Beis Din has overhead costs too (i.em electric, phones, maintenance etc).
So in this case, the money to the bais din would have been assur?
And if so, are any rulings that come out of a 'bribed' beis din also inadmissible?