Well, I think it depends on what's coming next. The ultrasound is to determine whether there's a problem with his liver, with the goal of medication or surgery. The ultrasound itself won't help or hurt anything. So if the u/s is unaffordable, the question is whether you could do the surgery if the u/s shows something, or daily medication if it's an infection or hepatitis. If the answer is no, then you should consider throwing some medication at him without the u/s (prednisone, for example), palliative care (pain relief), or euthanization (which is not a tragedy--better a short, high-quality life than a long, painful one). I wouldn't do the u/s if there's no way you could go beyond that.
By the way, pred is an appetite stimulant as well as a treatment for some liver diseases, and it's not SUPER expensive. You may want to try a few days of it to see if it turns him around.
We had a similar situation with a cat that we LOVED. He was sick and anorexic, but in his case his blood tests were normal. I did a week of prednisone and got him to eat for a few days (though he still acted very, very sick), but as soon as we weaned him off of it he went right back to hideous illness and anorexia. Our next step was u/s or x-ray and surgery--we opted to euthanize instead. It was incredibly painful, but trying to afford that surgery would have hurt our family more than losing our cat.