Most consultants charge if they incur an expense on your behalf. If you ask your consultant to call a software supplier in a distant area, you may be charged for the phone call. This is a kind of direct expense. When consultants must travel, most charge a per diem rate to go out of town. In expensive areas, expect to pay more.
Billing methods vary widely.
Daily and hourly rate consulting allows you the greatest control. It tends to be less expensive than fixed price contract because you assume more risk. The key to keeping expenses down is to use the consultant effectively. The section Getting What You Want shows you how. Some consultants reduce rates for long engagements.