Then you're doing it wrong. You see, this needs to be done after you already coded the application at least on 2 different versions and the codebase of it it's over 50k lines and at least a year old. Then implementing something fast it blows their mind especially when they have the proverbial carrot in that proverbial place. As for overpaying is not the case since I am paid hourly. I prefer to blow their mind from time to time in order to do repeat business instead to always be "I'll get back to you" and have them feeling that I'm procrastinating.