If your ghostwriter is sure of his skill and your case's merit, shouldn't he accept payment on contingency? Whether contingency payment is a good idea for you as customer depends on how much control over the product you want to cede to your writer. If you insist on exercising your right of control over content and style, then you cannot reasonably expect your ghostwriter to accept a contingency over which you exercise the greater control. If you are a lawyer, you will not want to give up your right of control, although if you made a wise choice for ghostwriter, you ought to follow his recommendations in most instances.
On the other hand, everyone responds to incentives. A professional may think he does his best regardless of the specific incentives, but this is illusion or self-deception. Making a minor part of the fee contingent provides an incentive for best performance, without impairing the ghostwriter's disinterest in the content.
What part of the fee do we perceive as significant but minor? From social practices that involve ascertaining a small but significant part — from tithing to tipping — treating 15% + - 5% of the fee as contingent should have a salutary effect.