I think it really depends on your niche. It's possible that, that is undercharging for some clients, but maybe that's overcharging for smaller clients. If you understand a specific niche really well and exactly what problems they have, how many calls they get, how much each call is worth, and how long it will take you to build it, firm pricing like that makes more sense. However, if you do not know a niche super well, I would start out with more custom pricing until you can figure it out. Just my 2 cents tho.