Kitty is always sick throwing up green bile!?

My cat is about 15 years old and lately he's sick everyday! He was sick as a kitty but not this much! We've got him when he was a runt and he was always a little fraile. But, now he throws up this dark green bile every day! Ne never goes out side. We've taken him to the vet only to pay 400$ every time we take him. We are at a loss. Any suggestions, pls help!