"Ok, I can help you with that. Mind if I ask you a few questions first?"
SFS
"Just about every situation is unique so can I ask you a few questions?"
Any job that is a 2000sqft house I will offer to do a pre-inspection.
Anything under that I can give a pretty good ballpark figure, letting the custy know that "IT IS A BALLPARK ESTIMATE AND I WILL BE TELLING THE TECHNICIAN THAT IT IS A BALLPARK ESTIMATE AND HE WILL GIVE YOU THE PRICE IN WRITING BEFORE HE STARTS."
If you don't let the custy know that you are talking to the tech then they will try to hold them to the low end price...yes even if you are the tech answering the phone and doing the job.
If they do want a ballpark then I always ask
Pet problem?
Furniture?
Moving it or going around it?
condition of carpet?
other concerns?
Why are they cleaning?
There are other questions that come up and the custy will usually give up info with having to ask.