Communicating variable pricing in a form field?

Scenario

I own a car rental shop and am setting up a fee schedule for when customers bring back a vehicle with signs of damage. Some damage has a defined price ie. a rear view mirror. Some damage is conditional on how severe it is, such as a dent.

Question

What is the best way of communicating the option to designate a variable fee versus a defined fee? A couple of options below:

Use hint text. Pricing remains variable, if value is not entered

Empty field. Validation message appears on mouse-out, if value not entered. Must use 0 for variable pricing.