>I hate to use the L-word, but you could negotiate a retainer like Lawyers do for those hours including a reduced hourly rate for what they actually use. Your job would be to predict how many actual hours you would have to work and base your retainer fee and hourly rate on that.

PMFJI

You lost me here. Are you saying that if you actually work the X number of hours you get you regular consulting fee (say $50 per hour), but if you don't work it then client gets a break at a reduced rate ($30 per hour)?