Most people nowadays seems to be using a repcal (which is pure calcium + vitamain D3) and herpvite, a broad vitamain supplement which does not contain vitamain A (which can be toxic to amphibians) but rather contains a precursor to facilitate its natural production by the frog's body. These supplements are used in equal proportions and frogs recieve them on most if not all food items.
The food debate is not as straight forward. I think that most people would say that some form of fruit flies that cannot fly are the most inexpensive option. I have raised frogs on a well supplemented diet of flies and have not had any problems, except for a while when I was a bit lax about supplementation.
I think that varying the diet is beneficial to the animals in general, but supplementation is more important.
Some foods that people use are the 2 commonly available wingless flies, D. melangaster and D. hyedii, termites (if you can get cultures or collect them from a guaranteed pesticide free local source), pinhead or week old crickets (generally an expensive option unless you culture them or have a large collection), Field sweepings (little bugs you catch in a net by running it through vegetation in an area you know is free of pesticides,herbicides etc), Confused flower beetles and a few others.
In my opinion the best way to go is make sure you have a consistent food source that is easy for you to use, supplement it with vits and cal, and use additional food sources when you can.
Hope that helps,
Aneides