Ok, that makes sense now. So you've pretty much ruled out a food allergy it sounds like. A lot of people here have managed symptoms like this in their cats by going to specifically a raw diet, which may be a varient of something you've already tried. If that is something you are interested in pursuing further, hopefully one of the many raw cat food people here will speak up.
If not, there are resources like catinfo.org and this facebook group:
https://www.facebook.com/groups/CatCrap/Otherwise, your vet may unfortunately be your best resource. Btw, I wouldn't think that blood tests would be a very expensive item. Ultrasound, sure, but blood tests don't normally break the bank do they?