I'd rather that part of my service is to give my client a sort of guarantee on the cost, given the requirements[1]. How many hours I work? What technologies I use? What OS do I work on? What's my text editor? What version control system do I use? These are things they shouldn't have to concern themselves with -- I'm a black box.

Do you ask Apple how much it cost them to make that iPhone? No!

[1] If the requirements change, then of course, my price will change (most likely increase).