Free Estimate at a Substantial Fee

We had to retain the services of a plumber recently due to water leaking under the house. After consulting the yellow pages, I selected three that advertised "Free Estimates". When we called, all three said the estimates were free however, there would be a "diagnostic fee" of $80. The explanation was that, if they had to look under the house, it went from a free estimate to a diagnostic fee. Huh?