How long are consumers willing to wait in order to find a bargain?
Check this paper out: 

The Value of Time Spent in Price-Comparison Shopping: Survey and Experimental Evidence
Author(s): Howard Marmorstein, Dhruv Grewal and Raymond P. H. Fishe
Source: Journal of Consumer Research, Vol. 19, No. 1 (Jun., 1992), pp. 52-61

With a bit of hand-waving, you can say that consumers are willing to search as long as value of time &lt; amount saved.