It is simply not true that senior lawyers in corporations view the law firms that vie for their work as interchangeable. At least for the work those lawyers understand well, as buyers they do not at all perceive the firms who do it as fungible. Law firms that do some other kind of work may appear to be all the same in capabilities, but that is ignorance and at the remove of 30,000 feet (See my post of March 6, 2007: law firms and fungibility; Oct. 5, 2005: myths cherished by firms; Aug. 24, 2006: Manhattan Project of GE; Oct. 30, 2006: little selection in specialized domains; and July 30, 2008: much legal work seen as commodity.).
Nor by any means do law firms as sellers picture clients as all of a kind. Each client opportunity is unique to the partner pursuing it, full of complexities, background, differentiators, pros and cons (See my post of Dec. 4, 2006: differences in what clients have to offer firms.).
Small wonder, then, that the costs of legal services roam all over the map. Standard supply-and-demand-curve economics assumes that all buyers are equal, and are the same in all important regards to sellers. Neither end of that classical assumption holds on the legal marketplace ground.