What is the test for "overwhelming" or "substantial" disparity in bargaining power between the parties?

It is not any inequality of position that will do. As Coutu J. observed in Floyd v. Couture, every contract involves some disparity between the parties in terms of bargaining power. It must be such that it has the potential for seriously affecting the ability of the relief-seeker to make a decision as to his or her own best interests and thereby allows the other party an opportunity to take advantage of the claimant’s personal or situational circumstances. That is why terms such as “overwhelming” or “substantial” or “special” have been used. While trying not to fall back into the linguistic trap I have eschewed previously, I would venture to say that what is meant by such terminology is that the inequality must relate to a special and significant disadvantage that has the potential of overcoming the ability of the claimant to engage in autonomous self-interested bargaining.