It is impossible to tell what the problem is based on the information you provided. You say the error says that the file is closed when you read the file. What file? Your code doesn't show any file (opening, closing, reading, or anything else). You need to find the relevant code and show us that, the code you showed here is irrelevant.
I guess the only thing I we can guess from the code you provided is that PriceBatchCreator is probably not thread safe and probably allows Objects to be shared between instances (such as via static variables, or some other mechanism.)
Even without knowing what your code is actually doing, this bit looks odd:
I get an error that the file is closed when I try to read the file with the second Thread after the first Thread has closed the file.
So, you're acknowledging that you've clsoed the file, and are trying to read from that closed file, but you're surprised that you're getting an error about trying to read from the closed file? The file that you said that you closed and are still trying to read from?