For loan-related signings, I look for the following:
The amount of faxbacks required
The distance traveled
The printing involved
If working with a signing agency, I aim for 90-100 for no faxbacks within a traveling radius of 15 miles. If working with a title company directly, I charge $150 minimum since there is no middle man. The price is increased with travel distance.
If it's not mortgage related, just ask the client to explain what is involved. If they want a quote (and most do), I give them a ballpark fee and am VERY clear it is likely to change (up or down) once I see the job. Everyone I've worked with has been OK with this approach.