As an author who's book (Riverwatch) is one of the initial five offerings for Unglue,it, I thought I'd stop by and add my two cents.
I see unglue.it as a way of giving something to the reading community that has supported my work for the last ten years, while at the same time earning some additional income for my creative efforts. It is not the LAST income I hope to earn with this particular title (as was suggested up thread) just one more piece of the puzzle to add to the initial rights sale and subsequent foreign sales.
In terms of how the financial goal for the campaign was derived at, I simply took what I earn from the work on a monthly basis from the various ebook distributors I've involved with and multiplied it by 60 - that would give me the income I could hope to earn off the work if nothing changed in the market. This is actually short circuiting myself a little bit, as I expected ebook sales to increase with time not decrease, but it was the most reasonable solution I could come up with.