My cat vomits

Question
For a little over 3 years, my kitty, Jack, has been vomiting on a regular basis.  We have followed veteranary advice to try different foods, wet/dry, name brand/off brand, to no avail.  He is current on his shots and we have 2 other cats, one is his mother, the other is a 18 month old that we saved from serious paw infection when he was abandoned and found at 6 weeks old.  When Jack vomits, it is usually either his entire meal or if he hasn't eaten, mucus.  Occationally there is fur, but it is not the rule, and we have put him on a hairball treatment over a year ago, also to no avail.  We are taking him to the vet on Monday, but I don't want to spend an exhorbant amount of money on tests if he may require serious treatment, as that gets to be pricey as well.  We will pay what it takes, but it would be nice to know where to start.  He is at least 5 lbs lighter than the other two cats, but doesn't show signs of emaciation or dehydration.  Please respond soon, I would greatly appreciate an opinion on this.  My most sincere thanks.

Answer
He most likely has a food allergy or inflammatory bowel disease.

I would recommend an all natural hypoallergenic diet such as those made by PRAIRIE (beef and barley) or Nature's Recipe (Venison & Rice).  Feed several small meals (3 or 4) throughout the day....do not leave food out all day.  Also supplement his diet with plain yogurt and digestive enzymes to improve digestive health.