We do not have personal experience with NG feeds. Our dr told us they use NG feeds at our clinic when it's anticipated that the supplementary feeding will only be needed on a very short-term basis; that when it's thought to be needed long-term they go with a g-tube.
When you ask about a cheaper enteral formula than Vivonex- many different supplements are used for enteral formulas, including some that can also be taken orally (i.e. Ensure.) Supplements like Ensure are much less expensive and can deliver the needed calories and nutrition. If what you mean is a less expensive elemental formula... I do not know. They are all very expensive and vary substantially in composition (percentage of fat, protein, how many calories, etc) ...you'll need the dr to recommend the right one for you. I don't know about getting the components separately; I haven't heard of that. Emily's Peptamen would cost about $700/month without insurance coverage so it's worth discussing with your doctor specific recommendations and trying to get coverage through insurance.