I cannot even begin to fathom any normal diagnostic tests that would cost $1,000? A senior profile at my vet clinic includes a physical exam, complete blood panel, urinalysis, and fecal exam is is only about $100.

LOL give me your vet's name. At my vet it's nearly 200. I'll drive to Atlanta!

To the OP, yes get urine tested for infection first. Bladder/kidney could cause lots of pain walking and picking up. Start simple.

Also, on the weight….I know folks here get tired of my saying it, but...
first treat for worms even if none show up in fecal. DO NOT USE OTC, get from vet. Then, order cyproheptidine. It is a cheap antihistamine that works miracles with increasing appetite. My basenji was getting to the bone stage, massive testing... all normal. She simply didn't care if she ate. No matter what your vet says, you don't go by AMOUNT for what is right, you go by results. If they eat and are thin, they need MORE FOOD or food with higher calories. Also, you might try something like FASTRACK, healthy bacteria which helps them USE the food they eat much better.