Here is some interesting language I came across a few days back. It might be helpful in certain cases.
No one questions that cross-examination to show the bias of a witness or his interest in a case is entirely proper. Impeachment of witnesses through a showing of bias or interest aids the jury in its difficult task of determining facts when it is faced with contradictory assertions by witnesses on both sides of the case. See generally McCormick on Evidence § 33 (1972). A pecuniary interest in the outcome of a case may, of course, bias a witness. Id. A showing of a pattern of compensation in past cases raises an inference of the possibility that the witness has slanted his testimony in those cases so he would be hired to testify in future cases. Collins v. Wayne Corp., 621 F.2d 777, 784 (5th Cir. 1980).