My ex wife makes more than I do. She lives with our 9 year old daughter and she stays with me over the weekend. I was giving her $500 every month for my child. But things have been getting tight lately and I won't be able to afford that amount on monthly basis. On the courts point of view, how much will it be fair for me to give her every month?

I take it that the $500. Is something you agreed on without any court orders since you are asking what the court would order.
Only a child support Judge can really answer that and it would also partly depend on your income and the cost of living of your area. Like a Judge in West Virginia might say you owe $500. A month while a Judge in California might say you owe $900. A month.
You really need to get a court order because if she decides to claim you owe for years ago and claims you haven't been paying then you will have serious problems.

There are many online support calculators to help in figuring child support. They only give an estimate. What state are you in ?