two very simple and reasduring reasons:

1. they work under a corporation with a strict hour set per day. IE 9-5. 8/hours a day. do the math.
2. A reputable/responsible person would record how long they work on each project. do some math.

this should be presented to them in the ever so obvious project break down. they should know you expect it to take X hours to complete. if they agree, it doesn't matter if you do it in one hour and make it up, they agreed ot pay you, so they have no reason to doubt you. however, you will most likely be screwed if you try this. i don't reccomend it, i'm just saying it's easy to reassure people.

my only questions is, why the variation in numbers? xhtml (regardless of doctype) isn't any harder to make than html, so how is it worth $10 more? your 'expenses' - to get a cms coded in 2 hours is more demanding thus a higher price, but adding / to the end of some tags doesn't justify that to me.