A finder's fee is a payment to someone, or a company, for their time in locating something for you. i.e. it is a fee for service.
Questions to ask yourself before hiring someone:
(1.) How much is your time worth to you?
(2.) How much time would it take you to find it on your own.
(3.) Does the finder charge by the hour or by the job?
(4.) If by the hour, multiply by their rate times the estimated number of hours. Also, if by the hour, does he have a max fee for the job?
Multiply your hourly time (1.) times the number of hours you think it would take you (2.) times 2 (because most people take twice as much as they figured it would take them to find something.)
If that figure is greater than the cost for the job, hire the finder and pay his fee.
I'm worth $70 per hour in total compensation to my company. However much your company spends to hire you is a good starting point for figuring what your time is worth (assuming you're not self-employed.)