As has been said, (this assumes the job and/or client are important enough to spend the time on), first confirm they are comparing apples to apples, if they're not then educate them, then if they are you can then make a judgement on whether you can discount the price any, if at all.
And for The Vector Doctor, cripes, who has the time to shop around to save $2 ??!!
I get people like that, they want a price on a set of mags and then tell me they have three other prices that are cheaper, obviously they eithe have more time than money, or more time than brains.