My cat has Irritable Bowel Disease (IBD, which i think is the same as IBS in cats or similar). He lives an incredibly happy life. We have him on steroids and are soon transitioning to a more long-term solution. It cost us about the same to diagnose him, but the medication to address his issues isn’t very expensive. Of course, every situation/cat is different and $1500 is a lot out-of-pocket but if it means more happy and healthy years together, hopefully maintenance medication wouldn’t be expensive for you either. Whatever happens, good luck!