Please. Who, in tech, is making $300 to $600 an hour "base salary"? Pretty much nobody other than a handful of VPs and CEOs. None of which are likely able to to do a real technical interview anyway.
$300 constitutes, before taxes, around $550k. This is not unheard of for senior / director level product management or engineering hires at some large tech firms. Do not believe what you read on glassdoor. There are plenty of Googlers that are making around $1M in salary in these types of roles.
To emphasize pmcgrathm's point, the $60/hr "level" is for "Management/C-level" candidate interviews. If you're paying a consultant to ask "the right questions," then at that level you're paying another management/C-level person, or a very experienced "C-level" headhunter/recruiter, and they're unlikely to do it for $60/hr.
That said, the same is true across all these pricing tiers. I wouldn't consult on a Junior level interview for $60/hr, much less $30. I can earn twice that as a W-2 contractor doing boring, unchallenging enterprise architecture and software.