Johnson v Robertson, 2015 NY Slip Op 06658 (2d Dept. 2015).
Moreover, “[p]roof of damages may be based solely on oral testimony as long as the witness has knowledge of the actual costs” (Electronic Services. Intl. v. Silvers, 284 A.D.2d 367, 368, 726 N.Y.S.2d 441). The record demonstrates that the Robertson defendants, who had 20 years of experience in construction and had built over 100 homes, had knowledge of the actual costs of the services being provided
You have someone who is close to an expert who tells you that it will cost $40,000 to fix a prior contractor’s defective work.  This testimony does not have an etiology in documentary evidence; it is solely based upon the contractor’s knowledge.  Enough?  Many judges will say no (true case I tried).  The Appellate Division said otherwise.