Here's the form I use. My holding fee is equal to the total deposit. Also, I won't hold for more that 15 days.
Non Refundable Binder
This will acknowledge receipt of the sum of $400.00 by Two Turtles Property Management from (applicant name) as a holding fee to hold vacant the rental property at _____________________________, until AUGUST 15, 2011.
The property will be rented to Applicant on a month to month basis at a rent of _________ per month, if Applicant signs Landlord's written lease and pays Landlord the first month's rent on or before that date, in which event the holding fee will be applied to the deposit.
Landlord and Applicant agree that if Applicant fails to sign the Agreement and pay the remaining rent and security deposit, Landlord may retain this holding fee to compensate Landlord for lost income and inconvenience.