My Cat Is Vomiting A Yellow Substance. He Sleeps All The Time. He Is Weak. He Won't Eat, But Drinks. He Has Lost A Significant Amount Of Weight? What Could Be Wrong?

I know this is not what you want to hear but this cat needs to be examined by a veterinarian.  The weight loss means this condition has been present for more than a few days--it is concerning when the body does not have further fat stores to use and starts burning muscle tissue.  Cats also are at a high risk for developing hepatic lipidosis (fatty liver disease) when they stop eating; this can develop even within a few days.
The most common problems in middle aged cats include kidney disease, liver disease, diabetes, hyperthyroidism, and chronic pancreatitis.
Just like with many things in life you get what you pay for--I am sure you could find a less expensive clinic that may not do a thorough workup or treat your cat appropriately.  The cost for similar care in human medicine is 3-10 times more expensive so it could be much worse.  You may also want to consider pet insurance to help with costs in the future.

I think, just think he ate grass.
Once, our dog in japan ate grass,and it Vomited a yellow Substance.