If the chance penalty was based on a percentage of the money you had on hand there would be no possible way you could be taxed into a negative balance, in less the tax was completely random with no set value allowing it to exceed 100%. Otherwise if you have $10 and your fined 100% you now have $0 if you have $10 and your fined 50% you now have $5 if your fined again lets say 50% you now have $2.50 but once you reach a balance of zero any percentage you are fined will equal zero (100% of $0 = $0)
Now that we have covered why you ARE NOT fined based on the money on hand let explain the 2 basic reason we should all know this by now.
1.) If all your streets are sabotaged and their rental value is Zero all chance penalties will also equal zero, thats right if you have no income value the penalty percentage reflexes that and the zeros cancel each other out and you are fined no money. Remember even 200% of Zero is still Zero (I Have experienced this first hand on more then one occasion)
2.) If that chance card penalty is based on your total rental income and not the money you have on hand it becomes much easier to explain the high fine values, if you setting on only $10 of the $100 you made but the percentage taken still reflects you having $100 then even small fine % appear to be bigger and can result in being fined into a Negative balance. Because we all know if you are fined 100% of $100 but you only have $10 on you well now your $90 short.
From the Official FAQ Section-
Q: I have a negative bank balance. What do I do?
A: If you receive a Chance Card that fines you more cash than you have in the bank, you will find your balance fall below zero. The quickest way to get back into the positive is to sell some of your streets to the bank! Unfortunately, for half their total value, but beggars can’t be choosers.
Please let me know if you have anymore question about chance card penalties, I can only hope that one day I will stop seeing uninformed post regarding how this work or that someone will provide some kind of proof that I am wrong ;-) :-)