Agree it is a difficult one to put a value on, not much to reseller but potentially high to end user- perhaps.
Just one point though- if I had worked on the other "extensions developed" basis I would have probably never have bought and subsequently never sold any names.
But I have- well over 1,200. Maybe 5% of those I have sold have subsequently been developed.
I think you will find that in the majority of sales most extensions have not been developed so not a good rule of thumb for determining valuations- all this of course is simply
IM-HO.com.