Yikes--$70 a day adds up fast! As far as I know, most insurance companies will cover formula costs for a child as long as NG or G tube feeding is needed. Some insurance companies won't cover ET if the child is drinking the formula (which seems crazy to me, since the child is saving the insurance company all the associated costs of bags, pumps, etc.) but I've never heard of one that didn't cover it for tube-feeding. Maybe it's because Boost Essentials isn't an elemental or semi-elemental formula?
It might be worth doing some investigations through your doctor's office to find out if there is a way to get the formula covered by your insurance, either by changing formulas, or changing the way the orders are written for the formula. (Sometimes it depends on if the orders are written as a prescription, or if the billing is done through a different category.) Does your doctor's office know that you are paying 100% for the formula? If the doctor's office can't help, I would try the dieticians at your local hospital, or whichever department is accustomed to setting up kids with tube feeding. They may have some advice for you.
When my daughter did ET via NG-tube, everything was covered 100%--all the supplies and all the formula. At first we had a local medical supply company, which we loved. The dietician there knew more about all the different formulas than the people at our doctors office, and he helped us find a formula that worked for my daughter when the first ones were making her really nauseated. After 3 months, we switched over to Coram, and they were fine too.
I hope that you are able to switch to a better company, and that you can figure out how to get the formula covered by insurance.