I would just give a high-enough-to-cover-the-worst-case estimate, and/or a rate to test for usefulness of the pre-wiring, with an explanation that the bill will reflect actual time.

If they say yes, you'll have a high-dollar job. If not, say thank you pleasantly, please call if you decide you want me to tackle the job, and leave your card.

If other installers back up your explanation, either before or after diving in, you'll look better for knowing ahead of time what lay ahead.

Sometimes, it seems like half of my work is a call back after the customer thanks, but wants a second opinion. I just look at it as a cold call where I already know the layout.

Larry
www.fineelectricco.com