Is it legal for my former attorney to write off $1000 in files at that time of the case and that the only way I can get a copy is to pay $500?

I don't understand your question. What do you mean by "to write off $1000 in files"? As for having to pay $500 to get a copy of your file, it may or may not be reasonable. Are you saying the case was handled over 15 years ago? If so why not just obtain a copy of the Decree from the Clerk of Court? Try that.