what is the best way to approach this
Anybody cheap is bad, ergo if it's way expensive it must be good? Not.
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It's the same as any other consulting. If you had the expertise you wouldn't need the consultant. So what do you do when you hire a lawyer? You trust him.
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Upon what do you base that trust?
- professional references/referrals
- education (school calibre) and work history (CV or resume or bio)
- gut instinct and impressions from meetings
- opinions of others knowledgable in related fields
- described process of billing/charging/planned work.
- perhaps issues related to jurisdiction and size of firm, etc.
SEO is no different. The fees are just as high. Retainers are common. Do it the same way.
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And as I do with lawyers, you can always hire a second one to audit the first one at least in the proposal stage. If you find a good one that is too busy to take your work (or is not interested because it's too low level) thay may agree to the consult for a fixed fee. They get a a chance to see someone else's work, and getpaid for their expertise alone (no work obligation).
If you do that make sure your contract with #2 permits you to share the proposal/plan with your "associates and hired expert advisors" or something like that so you don't do any harm.