Older editions of SR had a math equation to determine what a contact would charge a runner for his/her services. I haven't seen anything like that yet (unless I've missed it) in SR4. Has anyone devised anything, or do you just wing it?
Wing it. An equation could go like: AxDxC/L
A = availability of the information, similar to data search threshold
D = how dangerous the leaking information might be for the contact
C = contacts connection rating
L = contacts loyalty rating
Not for accuracy, but for displaying what has to be considered while winging.