I've read the paper you refer to, very interesting data indeed. The quote is one of five possible explenations of why the results differ so much, but it certainly is a good possibility.

This post sparked my interest/doubt knob for now. I will question more 'facts' in the SE world from now on.

About sommerville: Sommerville website: http://www.comp.lancs.ac.uk/computing/resources/IanS/

The book I refer to: http://www.comp.lancs.ac.uk/computing/resources/IanS/SE8/index.html

You can download presentations of his chapters here: http://www.comp.lancs.ac.uk/computing/resources/IanS/SE8/Presentations/index.html

I have based my findings on the presentations now, since I haven't got the book nearby. You can look them up yourself (download the chapters from the above link).

Chapter 7 says:

Requirements error costs are high so validation is very important • Fixing a requirements error after delivery may cost up to 100 times the cost of fixing an implementation error.

Chapter 21, refers to Software Maintanance, claiming (might need to verify this as well? ;)) :

[Maintanance costs are] Usually greater than development costs (2* to 100* depending on the application).

Because I don't have the book nearby I cannot tell for certain where it was stated. But I was pretty certain it was stated in that book.