There is schedule like this:
http://www.ultimatebet.com/games/rake.html
which says how much money they take from each pot.
For example, if you are playing at a full table, 3/6 holdem, the chart says "rake: $0.50/$10" and "cap: $3" which means they will take $0.50 when the pot reaches $10, another $0.50 when it reaches $20 and so on up to $60 when they will have taken the 6th installment of $0.50 for a total of $3. There is a cap of $3, so above $60 they don't take any more rake. If the pot is exactly 10, 20, 30, 40, 50, or 60, then they will have taken exactly 5%. If the pot is any other amount, they will have taken less than 5%
Most have a "no flop no drop" policy, so they only count the pot from the flop onwards.
Any questions?