Remember too, that even large companies may not have an expert in your field of operation. And you could have had me, or a dozen equally good others, for a fraction of the price. Ask if they can work within the budget . Know their track record. If other clients have had nasty shocks with the consultant's bills - don't even consider employing them.
Good consultants deliver on budget every time.
Be impressed by gadgets, jargon and sharp suits. All you should be impressed by are references. Not just written ones - talk to other clients who have used the consultant. Ask them to be brutally frank. Ask what it was like to work with the person, ask about their diligence, discretion, judgement and their capacity to sort out the wood from the trees.