I have been wondering this too. Are there really some frequencies that are better? When she says "moderately effective", she is saying that the resistance is quite a bit higher at 2 Hz than 5 Hz, which is in turn somewhat higher than the 20,000-30,000 Hz circuit she recommends. It's the capacitance of the body that makes the difference, by storing the electric potential for a certain amount of time, the shorter the better.
Hulda Clark says she used to use something like a Rife machine, and her detection mechanism (syncrometer) is frequency based. Then she discovered that a DC pulsing current wiped out every parasite she could detect. TOTALLY different principle. Only the ones hidden in shielded areas like the intestines (fortuitously including the natural probiotics we would die without), and within other parasites' corpses, remained. Harmful intestinal parasites have a nasty habit of digging into intestinal walls, so many of them do get killed with this approach.
Most people are intelligent enough to know that cost does not always equal effectiveness. If there are improvements that would make the $10 zapper more effective, chances are they wouldn't cost very much to make. Want 1,000 Hz? Or 1,000,000? Put a different sized resistor or capacitor in (zero extra cost!) and you've got it. It's not a cost issue, it's just a question of whether it'll do any good.