Be suspicious of a consultant who claims a shortcut to implementation that can be accomplished in three to five months. While this attribute may be the most difficult to evaluate after a 2-hour interview and a check of references, it is what separates a skilled consultant from a rigid, egotistical bore. Weaknesses in other traits can be overlooked if you can find that consultant with the ability to rapidly identify your organizational strengths and weaknesses, and apply the necessary firepower and resources to motivate your company to improve. He will perform specific in-house technical instruction, and not just a generic training session. Many times this philosophy comes with experience.
Personality also plays a part.
Consultant costs come in many shapes and sizes. Clearly establish how many on-site days will be required to complete individual tasks. By establishing a comprehensive plan, with detailed tasks and periodic milestones, the total cost can then be negotiated as appropriate. A clear plan, communicated to all parties, will prevent purchasing of needless services. No consultant worth his salt should attempt to document and implement a system in the conference room.